Providing support for metadata

This will provide metadaa support for various activities in the template
such as:

 - http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-attribute-metadata.html
 - http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-init.html
 - and others

Created objects:
 - `core.Metadata`
 - `core.Metadatums`
 - supporting tests
Added documentation for the metadata feature

This includes examples and references.
Version bump to v.0.1.2!

Changes:
- New MetaData types, see the complete package in the related pull request: https://github.com/seandst/cfn-pyplates/pull/10