Multi exchange broker mappings

[sandroboehme]

As promised, this is the PR for #7 "Enhance the broker mappings and store them for multiple exchanges". But it's a bigger one.
There one thing I'm not sure about: As I extend the Binance specific broker from CCXTBroker I wanted to avoid that it registers with the CCXTStore as well which is why I've prevented that with if name == 'CCXTBroker': like this:

class MetaCCXTBroker(BrokerBase.__class__):
    def __init__(cls, name, bases, dct):
        '''Class has already been created ... register'''
        # Initialize the class
        super(MetaCCXTBroker, cls).__init__(name, bases, dct)
        if name == 'CCXTBroker':
            CCXTStore.BrokerCls = cls

[Dave-Vallance]

@sandroboehme Amazing stuff... Will try to review after I look at the feeds

[armezit]

@Dave-Vallance Any chance for this pull request to be merged into the master?