fix connect and listenermixin combo (fixes #118)

The old code in connect dynamically imported the ListenerMixin methods, and threw an error if component already had prop with the same name that wasn't from ListenerMixin.

However, as React wraps all functions, the test to see if the methods were from ListenerMixin would always fail. This pull request therefore removes the test and replaces the error with a warning that you either clobbered your existing property, or used connect and ListenerMixin in the same object which is unnecessary.