Get rid of extra fields for failures within the Action layer.
Rip out failure fields from the validation layer.
Get rid of failure fields in documentation.
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.