add github repositories sidebar plugin

if you specify

github_user:

in you _config.yml and once you add asides/github.html to your sidebar
items, this plugin will fetch the specified users github repositories
and order them so the last pushed ones are shown first. Then it'll list
them in the side-bar, including a link and the repository description

The plugin will only list your own repositories, not forks, though this
might need to be configurable later
allow limiting the amount of repos to display

by setting github_repo_count to someething that's not 0, you can limit
the amount of repositories to render
provide an option to show or not show github profile link
make listing/not listing of forks configurable

some people have more or less taken over projects with their forks, so
it probably makes sense to still list them among the repositories
corrected fork skipping logic
fixed @extend for pp, code, and tt. The @extend was extending a non-existant entry.
page layout will now accept pages without title metadata
expand_urls now correctly matches urls pointing to the root directory, allowing / to be replaced with /subdir/ if site is configured with a different root directory
Added support for pagination with a blog index in any directory (helps with landing pages). Added configuration for pagination directory base
removed unusable blog_index partial
added blog_index_dir configuration and improved update_source
Codeblock regex improved to better detect extensions fixes #96, added support for tableizing non highlighted code blocks from liquid codeblock tag and backtick code blocks
reordered arguments in pagination
updated blog theme to default to left aligned images. To center images, use classname "center"
added stringex gem for permalink url generation
now only excerpted articles show the "read on" link. Also the text for that link can be set in the _config.yml
added excerpt_link to article include. fixes #97
centralized pygments aliases into pygments_code.rb. Added alias for .ru (ruby), which sort of fixes #108
Merge branch 'master' of github.com:imathis/octopress
added option to force syntax highlighting language, example: {% include_code file lang:ruby %}. Fixes #108