Allow subtypes for btceapi.keyhandler.KeyHandler
Added PPC/USD pair.
Merge pull request #25 from zku/subtyped-keyhandler

Allow subtypes for btceapi.keyhandler.KeyHandler
Added an explicit close() method to KeyHandler.
Added __enter__/__exit__ methods to KeyHandler so it can be used in a with statement.
Updated the compute-account-value.py script to use KeyHandler as a context manager.
Fixed compute-account-value sample to correctly handle open orders in currency pairs that did not include BTC.
Actually fixes what the previous commit purported to fix. :)