added ability to save b-factors with md.save_pdb

I wanted this functionality, so I added some code. I don't know if people are still interested in this, or if this is even the "right" way to implement it, but we can have the conversation at least. See #428 

It's worth noting that *all* of the fields are implemented in PdbStructure, so it seems a bit silly to re-implement everything rather than reuse that code. 
Replace with `raise ValueError`, and maybe use the better message from below, `"bfactors should be shaped like: (n_frames, n_atoms) or (n_atoms,)"`. Also, it often helps in debugging shape stuff if the error message mentions the shape of the input argument.