fix amino3to1 to handle multiple chain_ids

[rasbt]

The amino3to1 now correctly handles multi-chain PDBs and residue-number duplicates. The return value has been changes from a pandas.Series to a pandas.DataFrame as shown below:

from biopandas.pdb import PandasPdb
ppdb = PandasPdb().fetch_pdb('5mtn')
sequence = ppdb.amino3to1()
sequence.tail()

fixes #21

[coveralls]

Coverage increased (+0.3%) to 94.017% when pulling 43727ba on amino3to1 into da7668d on master.