Added VS2013 support

Bumped version to 2.5 and added 12.0 to the list of supported VS
versions.
Add GitHub stylesheets and scripts
Add implementation of IVsLanguageInfo for Markdown
Add rendering options page, allowing the user to select MarkdownSharp or GitHub for rendering the preview window (not yet enabled)
Updated highlight.js and mathjax.html to fix bugs
Support GitHub markdown rendering in the preview window (does not use repository-specific GitHub Flavored Markdown)
Make the GitHub authorization form larger to reduce the chance of requiring scrolling