lib/clients/websocket: use utility function for determining product IDs;

lib/orderbook_sync: accept multiple product IDs and maintain multiple product books;
lib/utilities: file for common utilities such as determining product IDs;
README.md: document changes to lib/orderbook_sync;
CHANGELOG.md: document chnages to lib/orderbook_sync;

Merge pull request coinbase#39 from jborseth/margin-endpoints

Add methods for margin endpoints

0.3.3-0

Merge pull request coinbase#36 from coinbase/version-0.3.2

0.3.2

0.3.1

0.3.0

0.2.2