Make Action.finish() a public API, allowing explicit finishing of actions.
Ignore more than one call to Action.finish(), otherwise following idiom
would result in multiple finish messages when the context is exited:

with action:
    try:
        doSomething()
    except OneError as e:
        action.finish(e)
    except Exception as e:
        writeTraceback(logger)
        action.finish(e)
Document the new API.
Address review comments: Slightly better docs.
Merge branch 'python3-1' into explicit-action-finish-12
News entry.
Merge pull request #10 from hybridcluster/python3-1

Python 3.3 support.

Fixes #1.
Merge pull request #39 from hybridcluster/explicit-action-finish-12

Public Action.finish() API.

Fixes #12.
Merge remote-tracking branch 'refs/remotes/origin/master' into no-failure-fields-14

Conflicts:
	docs/source/usage.rst
	eliot/_action.py
	eliot/tests/test_action.py
Fix failing test.
Fix typo.