Add support for title and description metadata

[LukeMarlin]

• Refactor _resolve_additional_properties to reuse parts of it for title and description support
• Add _get_schema_metadata that retrieves the 3 currently supported metadata options on a schema
• Rename 'cls' params to something less confusing as 'cls' is a python keyword
• Add documentation for title, description and additional_properties
• Move addtional_properties tests to a new forlder "schema_metadata_tests" so they can be gathered in a common place

Closes #41

[coveralls]

Coverage decreased (-0.8%) to 98.273% when pulling 8f38e0f66a7affcef7a0ac7d4c5fdc709cd78eb1 on LukeMarlin:more_metadata into 3c579c7d3b9c579e8610f7e698d4589604d4884c on fuhrysteve:master.

[LukeMarlin]

I'm unsure what to do with the CI issues. I can apply black's suggestions, but I don't know why the build really failed. Also, I'm pretty sure that the coverage is incorrectly computed, as we can see in #98 , it decreased while only documentation was touched :(

Any pointers @fuhrysteve ?

[ksbrar]

Bump - any update on this?