JSON schema draft 7 support

[atmo]

This adds support for the latest version of JSON schema, draft 07.

Notable changes:

• Raise UnsupportedValueError instead of ValueError for Length and Range validators if they were passed with fields which are not supported.
• Empty enums are now allowed for OneOf validator. Quoting spec: "The value of this keyword MUST be an array. This array SHOULD have at least one element. Elements in the array SHOULD be unique."
• Proper "$schema" is included into generated JSON schema.
• Support min_inclusive and max_inclusive from marshmallow-3.0.0rc7.
• Marshmallow 3 is no longer supported for Python 2 as supporting both marshmallow 3.0.0rc5 and the latest version (rc8 as of now) requires too much effort with no apparent benefit.
• additionalProperties is always included in generated schema as with changes from additionalProperties support #77 it is always possible to unambiguously detect its value.

I suggest supporting only the latest draft and add compatibility for other versions only if there is enough demand.

[coveralls]

Coverage remained the same at 100.0% when pulling bf22eb7 on atmo:draft7_support into c65d473 on fuhrysteve:master.

[fuhrysteve]

Draft 7!!! 😲 Excellent code and great tests! Thanks for the pull!!