Added backreferences to `Seqeunce`s from `Monomer`s (closes #49) WARNING! Please note that `Sequence` API almost changed entirely! WARNING! This commit immediately obsoletes classmethods `Monomer.from_code*`, `Monomer.from_name` and `Sequence.from_monomers`. Turns out, python can not pickle sets/dicts which have keys, which inderecly reference the set/dict itself: http://bugs.python.org/issue9269 -- which is excatly what we have in abundance after this change. To allow pickling added `__getstate__` to `Monomer` to return all attributes, except `sequence` and `__setstate__` to `Sequence`, which runs through all monomers and returns the `sequence` attribute back to where it belongs. WARNING! This MAY result in unexpected behaviour in some cases. (Which should be rare enough).