Sketch of the API.
A better, more consistent method name for action finishing.
Hookup DeferredContext.addCallbacks to the underlying Deferred's addCallbacks.
DeferredContext.addCallback calls addCallbacks.
DeferredContext.addErrback and addBoth implementation.
Callbacks added to DeferredContext are now run in the context of the action.
Callbacks added to DeferredContext have results passed on, just like
regular callbacks.
Start implementation of addActionFinish.
Finish up DeferredContext.addActionFinish.
Fix erroneous reference to finishAfter.
Get rid of references to Action.finishAfter.
Get rid of references to Action.finishAfter.
Update documentation to cover new Twisted APIs, and improve organization
of action section slightly.
News entry.
Skip tests when Twisted is unavailable.
Merge remote-tracking branch 'refs/remotes/origin/master' into better-deferred-api-13

Conflicts:
	docs/source/usage.rst
Try to explain better the motivation for ``DeferredContext``.
Merge branch 'better-deferred-api-13' into trial-logs-69
Start implementation of redirecting Eliot logs to Twisted.
Tests for actual output of Twisted log messages.