v0.2.0

Completely rewrite to improve all the things.

• Resources and Collections now contain data and are responsible for serializing it when they are converted to JSON-API output (whereas before serializers were responsible for creating concrete Resources/Collections containing pre-serialized data). Serializers are now only responsible for building attributes and relationships. This is a much more logical/testable workflow, and it makes some really cool syntax possible!
• Support for sparse fieldsets.
• Simplified relationship handling.
• Renamed Criteria to Parameters, add validation.
• Added ErrorHandler for serializing exceptions as JSON-API error documents.
• Updated docs.
• Wrote some tests.
• It should go without saying that this is not at all backwards-compatible with 0.1.