Sche-ma Sunday

This release adds support for:

• OpenAPI server URLs directive #129, thanks @bombaywalla
• OpenAPI yaml definitions #132, thanks @bombaywalla

This release fixes:

• Incorrect errors of disallowed keys for OpenAPI bootstrapped instances #126
• The internal martian spec of an input schema to allow optional keys #138, thanks @chopptimus
• OpenAPI parameters defined in a $ref #112, #142
• Nested objects with optional keys in Swagger bootstrapped instances #111
• Interpretation of the number leaf schema #146, thanks @bombaywalla
• Interpretation of untyped schemas that have properties as objects #148, thanks @bombaywalla

This release improves:

• The main readme and the VCR readme, thanks @rgkirch