Provide a way to disable asynchronous execution

Currently, it's a royal pain in the butt to debug Concert because everything can potentially run asynchronously and the raised exceptions usually do not tell where they happened.

This change adds the module variable `concert.asynchronous.DISABLE` which, when set to `True`, will create fake Future objects which are the same as real Futures except that they aren't submitted to any kind of executor but return immediately with a valid result. Hence it's much easier to reason about the flow of execution.

To make good use of this feature I added a nose plugin, that is installed with the custom runner `runtests.py`. Note that it takes the same options as if you'd call `nosetests`.