Market improved and keyring support added to wallet

* Dokumentation for several functions of market improved
* dict keys for ticker renamed to small names width underscore (identical to python-steem)
* trade_history added. This function allows to fetch a fixed number of trades at each intervall to reduce the time. E.g. it is possible to receive the from the last 7 days 100 trades each 6 hours.
* utc time transformation is added to trades
wallet
* keyring can be used to unlock the wallet. Please store the masterpassword with  python -m keyring set beem wallet