Add validate method to xblock core.
Merge pull request #250 from edx/christina/xblock-validation

Introduce validation method and class in XBlock API
Consistently use empty instead of is_empty
Merge pull request #254 from edx/christina/validation-empty-cleanup

Consistently use empty instead of is_empty
Move xblock-family mixins into a separate python module
Extract Runtime-services machinery from XBlock into a mixin
Add a test that passing a FieldData to the XBlock constructor stores it in _field_data
Make XBlocks and Fields access FieldData via a runtime service.

Maintains the ability to pass a FieldData in XBlock constructors for
backwards compatability.

[PLAT-206]
Deprecate passing field_data through the runtime constructor