Added Attribute and split the classes into their own files.
WIP. May not work
Added Attributive and Attribute
A whole bunch of refactor
Added _id instance method and fixed document type detection
Added required check
Added more tests and some minor refactor to make timestamp more consistent
Added has_default and fixed required and default directives behaviors
Replaced id format to use underscore so it's more friendly as path in URLs
Bumped version to 0.4