Add ability to download new versions of KEGG

[hbsmith]

Probably through Biopython's Togows module to grab easily parsable json files. This is necessary prior to additional annotation (e.g. adding information on reaction balancing or free energies).

[hbsmith]

I'll probably just copy and adapt code that I wrote from here: https://github.com/ELIFE-ASU/ecg/blob/master/ecg/kegg.py