Only have the latest version in one place

From now on, anything that wants to reference the most recent version of SymPy
should import sympy and use sympy.__version__.  Anything else should be
assumed to not update when we release.
Merge branch 'master' into 0.7.3
Fabric: get the version number automatically

Also, build the pdf docs
Improve comments
Fabric: checkout a branch
Allow to specify a branch

This finishes my changes from the previous commit
Enable installing docs packages
Merge pull request #2253 from sympy/0.7.3

0.7.3