Add support for minProperties, maxProperties

[Tatsh]

minProperties and maxProperties are for objects and are mentioned here: https://github.com/json-schema/json-schema/wiki/ChangeLog
http://json-schema.org/latest/json-schema-validation.html#anchor54

This looks like it would stop this from working (which is good in some cases):

Schema:

{
  "schema": "blah-url",
  "type": "object",
  "properties": {
    "a": {
      "type": "boolean"
    }
  },
  "maxProperties": 1
}

input JSON:

{"a": true, "junk_field": 2}

Error would be on the junk_field being an extra and maxProperties being exceeded.

[mchiocca]

Hello. minProperties and maxProperties were introduced in draft-04, which is only partially implemented by this JSON Schema Validation implementation. Refer to #43, which adds the full draft-03 and draft-04 test suites. We need draft-04 compliance as well so we will be working to add missing features in the near future. So, look for these to be implemented quite soon. Thanks!