Extract category_link filter from category_generator.rb plugin

Allows category_link to be used as a filter to display a link to
any category.
Added robots.txt, with sitemap link
Updated Github API to V3.

Github API V2 has been removed.  The response data format also changed.
Updated Github API to V3.

Github API V2 has been removed.  The response data format also changed.
Fix for issue #622

The filters bundled with Octopress are not applied to files ending in 
`.md`, because the match in octopress_filters.rb does not match `.md`.
Merge pull request #706 from Confusion/patch-1

Fix for issue #622
bumped rbenv version fixing #743
Merge branch 'master' of github.com:imathis/octopress
Updated Github API to V3.

Github API V2 has been removed.  The response data format also changed.
Updated Github API to V3.

Github API V2 has been removed.  The response data format also changed.
Merge pull request #675 from zlu/master

Update Github API to V3
Twitter API call and links are now protocol relative
Merge branch 'master' of github.com:imathis/octopress
Using https instead of protocol relative urls, for Pinboard and Twitter feeds
Disabled Rsync delete by default. Closes #787
To support url without domain name like /path/file

You need to know if you would like to support this, each word in the title can not start with "/".
Fixing English error pet-peeve.
Merge pull request #831 from parkr/english

Fixing English error pet-peeve.
Rakefile option for any additional rsync arguments

Added variable `rsync_args`
gist tag plugin now works with the latest changes GitHub gists and does not fail if you do not specify a filename.