Support yaml metadata for actions and rules

* Add: Yaml support for actions and rules meta. Both yaml and yml extensions
are supported.
* Add: yaml, yml and json meta for sample action in fixtures for testing.
Add: Support for defining actions in yaml
Merge pull request #782 from StackStorm/STORM-653/yaml_support_for_actionchains

WIP: Add: Support for defining action chains in yaml
Introduce a MetaLoader class for handling meta

* Add: MetaLoader class that can load content from json or .yml/.yaml.
Switch all registrars to use MetaLoader
Fix: Explicitly use json.load and yaml.safe_load parse methods
Fix: yaml safe_load throws ParserError
Fix: Misnamed method; change argument names
Fix: Log runnertype when runnertype not found.
Merge pull request #781 from StackStorm/STORM-653/yaml_for_artifacts_meta

RFR: Support yaml metadata for actions and rules