Fix the help message for -c option (regarding chains w/o ID)
Use get_titratable_groups() instead of manually building up this list

in find_covalently_coupled_groups().
Added --titrate_only option and tests for it.

This option makes it possible for PROPKA to treat only a subset of protein
residues as titratable.
Merge pull request #6 from schrodinger/titrate_only

Titrate only option