Update index.md

Add link to OWIN / Katana Web API with F# article.
Update Web Programming guide.
Add reference to F# Web Stack group.
Link F# Web Programming Group to F# Web Stack Google Group.
Change 'OWIN and Web API' to 'OWIN and Katana'
Merge pull request #293 from panesofglass/gh-pages

Update Web Programming guide.