Messages Per Validation

* I added the option to pass a dictionary object into `check` so you can specify custom error messages for each validation that is in use for the check.
* Updated README
* Added unit tests to prove it works

I think the version should be bumped up in "package.json", but I don't know how you've been doing versioning so I am open to suggestions.

I'm also not sure if the client side code is still being maintained, but I only made the change in the Node version. I didn't know what the procedure was for minifying the client side version.

Fixes #187