0.3.0

This release introduces the following changes:

• Enactments may choose text by section without a TextQuoteSelector
• Remove "regime" parameter from Enactment
• Add data serialization using Marshmallow
• Migrate JSON data loading functions to Marshmallow
• Add Decision class containing Opinions
• Add Explanation class to clarify relationships between Holdings
• Improve readability of string representations of objects
• Move text selectors to separate anchorpoint library
• Add apispec schema specification for Holding input JSON files