update README for OS X installation

distutils does not play nicely with c++11.  A hack is needed to install
it on OS X.  This details how to do it.
fixes for new versions of networkx

In newer versions of networkx the connected_components algorithms
implemented as generators rather than returing lists.  These changes
make pele play nice with both versions.
Merge pull request #40 from js850/new_networkx

fixes for new versions of networkx
Merge branch 'master' of github.com:pele-python/pele into frozen_hs