Enhance Omittable mixin to accept '@default' keyword

The main idea is to use for initial validation of default
value the same code that is used for initial validation during
`create()` call.
Merge pull request #11 from tsufiev/defaults

Enhance Omittable mixin to accept '@default' keyword
Update @default to accept functions, adding @default unit tests
Merge pull request #12 from rackerlabs/defaults

Defaults