PR: add pyalpm.vercmp as alternative to parse_version

`pkg_resources.parse_version` has the different result from
`pyalpm.vercmp`. This change to github repos allows to use
`pyalpm.vercmp` as sort key.

This is just a PR for review comments. If this PR is accepted,
I will further implement this option in other repos and core.

README.rst

@@ -180,6 +180,11 @@ ignored_tags
   be useful to avoid some known badly versioned tags, so the newer tags won't
   be "overridden" by the old broken ones.
 
+sort_version_key
+  Sort the version string using this key function. Choose between ``parse_version`` and 
+  ``vercmp``. Default value is ``parse_version``. ``parse_version`` use 
+  ``pkg_resources.parse_version``. ``vercmp`` use ``pyalpm.vercmp``.
+
 An environment variable ``NVCHECKER_GITHUB_TOKEN`` can be set to a GitHub OAuth token in order to request more frequently than anonymously.
 
 Check BitBucket

nvchecker/source/github.py

@@ -5,6 +5,29 @@ from functools import partial
 from pkg_resources import parse_version
 from tornado.httpclient import AsyncHTTPClient, HTTPRequest
 
+try:
+  import pyalpm
+  def cmp_to_key(mycmp):
+    'Convert a cmp= function into a key= function'
+    class K(object):
+      def __init__(self, obj, *args):
+        self.obj = obj
+      def __lt__(self, other):
+        return mycmp(self.obj, other.obj) < 0
+      def __gt__(self, other):
+        return mycmp(self.obj, other.obj) > 0
+      def __eq__(self, other):
+        return mycmp(self.obj, other.obj) == 0
+      def __le__(self, other):
+        return mycmp(self.obj, other.obj) <= 0
+      def __ge__(self, other):
+        return mycmp(self.obj, other.obj) >= 0
+      def __ne__(self, other):
+        return mycmp(self.obj, other.obj) != 0
+  vercmp = cmp_to_key(pyalpm.vercmp)
+except ImportError:
+  vercmp = None
+
 GITHUB_URL = 'https://api.github.com/repos/%s/commits?sha=%s'
 GITHUB_LATEST_RELEASE = 'https://api.github.com/repos/%s/releases/latest'
 GITHUB_MAX_TAG = 'https://api.github.com/repos/%s/tags'
@@ -15,6 +38,7 @@ def get_version(name, conf, callback):
   use_latest_release = conf.getboolean('use_latest_release', False)
   use_max_tag = conf.getboolean('use_max_tag', False)
   ignored_tags = conf.get("ignored_tags", "").split()
+  sort_version_key = {"parse_version": parse_version, "vercmp": vercmp}[conf.get("sort_version_key", "parse_version")]
   if use_latest_release:
     url = GITHUB_LATEST_RELEASE % repo
   elif use_max_tag:
@@ -26,15 +50,15 @@ def get_version(name, conf, callback):
     headers['Authorization'] = 'token %s' % os.environ['NVCHECKER_GITHUB_TOKEN']
   request = HTTPRequest(url, headers=headers, user_agent='lilydjwg/nvchecker')
   AsyncHTTPClient().fetch(request,
-                          callback=partial(_github_done, name, use_latest_release, use_max_tag, ignored_tags, callback))
+                          callback=partial(_github_done, name, use_latest_release, use_max_tag, ignored_tags, sort_version_key, callback))
 
-def _github_done(name, use_latest_release, use_max_tag, ignored_tags, callback, res):
+def _github_done(name, use_latest_release, use_max_tag, ignored_tags, sort_version_key, callback, res):
   data = json.loads(res.body.decode('utf-8'))
   if use_latest_release:
     version = data['tag_name']
   elif use_max_tag:
     data = [tag["name"] for tag in data if tag["name"] not in ignored_tags]
-    data.sort(key=parse_version)
+    data.sort(key=sort_version_key)
     version = data[-1]
   else:
     # YYYYMMDD.HHMMSS