ENH: atom names, residue names and residue numbers added to the pdb writer.

This is a very minor change. Just added more information in the pdb writer from atoms.arrays. It is the same information that can be obtained from the reader (residue names, residue numbers and atom names). In the case this information is not in atoms.arrays, the pdb file would be exactly the same as it was done so far. I was cautious to keep following the pdb convention (http://www.wwpdb.org/documentation/file-format)

This is important when the user is working with proteins for example and wants to generate a pdb to use with other codes like gromacs.