Improve dialog handling: Drastically reduce latancy.

Improve dialog handling: Drastically reduce latancy.
 + Make FinTSDialog re-entrant: Entering the context multiple times will only init()/end() at the outermost layer
 + Turn FinTSClient into a context manager. Entering it will start a dialog that will be kept open, no need to repeatedly start/end dialogues

Example:
````
f = FinTS3PinTanClient(...)
with f:
    accounts = f.get_sepa_accounts()
    balance = f.get_balance(accounts[0])
    statement = f.get_statement(accounts[0])
````