Fix invalid atom feed id:tag

Fixes #1039: The atom feed generated produces an invalid attribute
id:tag when SITEURL has something different that http or it contain
a subdirectory, such as:
  * https://myblog.com/
  * https://mysite.com/blog
  * http://mysite.com/blog
Merge pull request #1140 from gpoo/fix-invalid-atom-feed-uid

Fix #1039 Atom feed for sites using HTTPS or a subdirectory
Build Pelican wheels in universal format

Including a binary wheel when uploading Pelican to PyPI will speed up
installation for the Pelican community. This change ensures that the
Pelican wheel is compatible with all supported Python versions. For more
information, see: https://pypi.python.org/pypi/wheel