added ability to save b-factors with md.save_pdb
Merge branch 'master' of github.com:schwancr/mdtraj into bfactor
moved bfactors kwarg in PDBFile.write to last.
small changes to exceptions and what not
added test for bfactor code
Merge pull request #556 from schwancr/bfactor

[ENH] Ability to save b-factors with md.save_pdb