Implement Action.context(), a context manager that doesn't actually
finish the action.
Merge branch 'explicit-action-finish-12' into non-finishing-context-17
Document Action.context().
Merge remote-tracking branch 'refs/remotes/origin/master' into non-finishing-context-17

Conflicts:
	docs/source/usage.rst
	eliot/_action.py
News entry.
Merge pull request #40 from hybridcluster/non-finishing-context-17

Action.context(): context manager that doesn't finish the action.

Fixes #17.