Added necessary hooks for monomer pickling [closes #35] Current implementation is lazy and does not store all monomer classes explicitly in some module. They are still generated on the fly. Some monomer classes have the same name as per PDB database. In order to avoid name clashes, we add underscores to classes, if same name class already exists. WARNING. This may and WILL cause trouble, if such clashes occur between different types of monomers, in which case different names will be generated for the same class depending on the order of loading modules. The only example of such clash in the current database is dna monomer "0AV" and rna "A2M", which both have name "2'-O-METHYLADENOSINE 5'-(DIHYDROGEN PHOSPHATE)"