Make Field, MessageType and ActionType descriptions optional.
Merge branch 'no-failure-fields-14' into simplified-types-2

Conflicts:
	eliot/_validation.py
Add fields to the public API.
Implementation of fields(), a shorthand for creating Field lists for
MessageType and ActionType.
Documentation for the new API.
Merge remote-tracking branch 'refs/remotes/origin/master' into simplified-types-2
News entry.
Address review comments: Tweak documentation.
Merge pull request #37 from hybridcluster/simplified-types-2

Simplified type creation by adding a `fields()` utility function.

Fixes #2.