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
Added Plotly to "Working Environments" and "tools with a (mostly) 2-D focus"
added NIFTY under Bayesian Statistics
Merge pull request #40 from mselig/patch-1

added NIFTY under Bayesian Statistics
Merge pull request #39 from mattsundquist/patch-1

Added Plotly to "Working Environments" and "tools with a (mostly) 2-D focus"
Merge pull request #31 from pv/splitnav

Split scipylib to a more clear sub-site
MAINT: make capitalization (NumPy, SciPy) consistent.