Fix #46. Add support for draft v4 minProperties and maxProperties.

Addresses issue #46. Implements support for `minProperties` and `maxProperties` per the draft v4 specification. Removed tests for these attributes from the list of draft v4 tests that are to be skipped so that they are included when running the draft v4 test suite.

@justinrainbow Looks like my request includes your last two commits. Probably because I did a git merge instead of a git rebase. I hope that's not a problem. Please consider merging this request since it implements important functionality and closes out an issue. Thanks!