Added documentation; made small changes to site

I added a docs/ directory inside nmatrix/ and updated its index.markdown page. Also, I removed the direct links to Github from the navbar and put them inside the sciruby.com/nmatrix page.

One *small* change that I made is to replace "Documentation" with "Installation" in the navbar, after all there's only installation information there.
Merge pull request #2 from agarie/source

Added documentation; made small changes to site