Add json-schema->prismatic transformation support

[jetmind]

Hi, it's a great library you have here! I want to suggest supporting JSON Schema transformation.

This is only one way support for now, but still should be useful.

Supports:

• all JSON Schema types
• nullable types
• union types
• enums
• arrays as lists and tuples
• min/max and uniqueness validation for objects and arrays

Does not support (yet):

• validators for primitive types (int in range, string length, etc)
• patternProperties for objects
• dependencies & schema references
• schema combinators

Looking forward to hear what you think, thanks