Merge lp://staging/~free.ekanayaka/txamqp/amq-endpoint into lp://staging/txamqp

This branch adds an AMQEndpoint class which has the same interface as a regular IStreamClientEndpoint, but with some more assumptions about the factory being passed and the transport to be used.

Example code:

from twisted.internet import reactor
from twisted.internet.defer import inlineCallbacks
from txamqp.factory import AMQFactory
from txamqp.endpoint import AMQEndpoint

@inlineCallbacks
def connect():
    endpoint = AMQEndpoint.fromURI(reactor, "amqp://guest@:guest@localhost/")
    client = yield endpoint.connect(AMQFactory())
    # ... "client" is a connected and authenticated AMQClient

For now no support for TLS connections is in place, but it'd be easy
to add as follow-up branch, if there's desire for it.