Actions validation fixes

At the time the Charm's Actions schema is loaded from YAML, gojsonschema
now uses its Validate method to ensure that the loaded schema conforms to
JSON-Schema Draft 4. The schema for JSON-Schema Draft 4 has been added
as an on-demand loaded definition to the charm package in order to avoid
remote queries to the spec.

This PR is a work in progress and needs discussion.  See https://github.com/juju/juju/pull/63#discussion_r13601842 and http://paste.ubuntu.com/7625313/