Ready to document and firm up the API.

Starting with a reset.
Adding the inflector, which we'll use heavily.
Adding the entity back.
Add back the manager for creating types.
Add the attribute manager.
Adding the core of Hum models, the metadata.
Adding documentation for metadata.
Adding the three attribute types.
Allow configuration of models with `configure`.
Adding docs for defining models.
Fixing a few documentation typos.
Adding documentation for defining attributes.
Add static method for getting metadata instances.
Reset metadata every test run.
Adding the mapper classes for mapping models to entities.
Add generic support for the basic PHP types.
Fix an invalid class name.
Don't access array keys that might not exist.
Add the Attributable trait.
Document Attributable.