A wrapper around gatherResults which logs failures from a list of deferreds - fixes 296

Here's another design sketch.

I'm not sure if this is the generic solution that @itamarst had in mind when we were discussing this in https://github.com/ClusterHQ/flocker/pull/563#issuecomment-52213838

There are actually a bunch of Twisted tickets proposing improvements / replacements to DeferredList and gatherResults. 

I thought about how I might address some of those, but nothing really sprang to mind.

 * http://twistedmatrix.com/trac/ticket/5061
 * http://twistedmatrix.com/trac/ticket/4977
 * http://twistedmatrix.com/trac/ticket/4919

The discussions in those tickets seem to suggest that there should be a suite of `gatherResults`-like functions for different use cases. I guess the logging of errors is one such use-case but a function called `gather_deferreds_and_log_errors` seemed a bit long winded.

I tried to make the logger configurable, with a view to using Eliot logging in the future, but I guess might just write another wrapper for that.

Anyway, up for design review and discussion.
This test only tests that the errors are consumed, since the `DeferredList` returned by `gatherResults` keeps a reference to all the deferreds pass to it (thus keeping them from being garbage collected). If you add `del d` before flushing errors, the test will still pass, even though the errors are logged during garbage collection.

The test would be more robust if it kept a reference to the deferreds that have errors that we want consumed until after we have flushed logged errors.