Add KEGG API Querying Support

This adds support to query KEGG's REST API (http://www.kegg.jp/kegg/docs/keggapi.html) along with simple tests which ensure that the correct url is hit and documentation in the cookbook. 

This has been discussed on the mailing list in the following thread: http://lists.open-bio.org/pipermail/biopython-dev/2012-October/009981.html.