Return `$this` for all set methods

• Thing::_set now returns $this
• Add typehinting for all set* methods

Test database & PHP classes match

• Move all testing code to .tests/ directory
• Add tests to connect to and retrieve table list from database
• Check that a database table exists for each PHP class
• Check that a PHP class exists for each database table
• Add missing tables for ListItem & QuantitativeValue

Class & table consistency

• More new classes & tables
• CreativeWork now has all props (some commented-out)
• All tables StudlyCase
• All props/columns now camelCase
• Properly quote all tables & columns in SQL
• Misc. bug fixes

Basic limited functionality

This is essentially functional but should not be used

• Add more classes & tables
• Add some missing properties
• Database structure should now nearly match classes
• Implement basic PHP linting
• Add PostgreSQL to Travis-CI config
• Fix various typos and other mistakes

Alpha 0

• Basic structure & documentation
  This is an early alpha and is not suitable for use of any kind