removed all references to internal asset_ids

-not tested
-need to change interface of oc_market_gateway.on_market_data_update
method
removed get_position_by_asset_id method
removed persistence.py this should eventually live in an entirely different service
Removed all references to internal asset ids

-renamed some methods to reflect the fact OCMarketGateway will now only
work with Options City id's
-added instrument_id to class_id lookup dict and lookup method
Changes to reflect method name changes in OCMarketGateway
removed deprecated api_proxy module
changed all public_api methods to refer to instrument_ids instead of asset_ids