Added a KEGG API Wrapper, docs, and tests

Take into account Peter's suggestions in PR#152. Move functions out of
KEGG/__init__.py and rename them to include "kegg_" prefixes. Change
function param names to better reflect KEGG endpoint params. Update
interface to include new KEGG functionality. Attempt to add more
argument validation with richer error messages.
KEGG get also supports kgml output
PEP8 tweaks to KEGG REST code
KEGG REST API tests using unittest.

Based on the print-and-compare tests prepared by Kevin Wu
(which I did not commit), plus an example using KGML originally
using Leighton's KGML_scrape.py for online access.
Thank Kevin and mention KEGG REST API in NEWS file.

This closes PR #152.
Basic docstrings for KEGG REST API.