Add mutually_exclusive params validation

I'm working on a project using Grape that validates mutually exclusive params quite a bit, and while digging around for an elegant solution, found [somebody asking if there was a way to do it through the dsl](http://stackoverflow.com/questions/22369525/how-to-validate-mutual-exclusivity-of-params-in-grape-api-ruby).

Seems possible to me, thought I'd take a stab at it. Does this work?

ps noob to Grape's code-base, let me know if I'm doing something stupid :)