Split navigation to separate "scipy stack" and "scipy library" parts.

This makes the Scipy library pages a sub-site of the main site.
Bias the top-level site less towards Numpy+Scipy
scipylib: replace the 'distributions' section with a link to scipy stack install instructions
scipylib: fix build instructions navigation
Replace Scipy -> Scipy library in a number of places
Merge pull request #31 from pv/splitnav

Split scipylib to a more clear sub-site