excempt models from schema validations

[juliankrispel]

I am using third party gems (namely paper_trail) which break when using this gem.

I'd like to blacklist models rather than attributes. Would accept a pull request that implements this?

[juliankrispel]

cc @ronen

[ronen]

@juliankrispel sorry for the slow reply. Sure, I'd be happy to accept a PR for that.