Support for combined experiments (see README)

[daviddening]

This pull requests proposes allowing users to create combined experiments, so they can see how one change affects multiple behaviors (ie does changing the button size increase its clickability at the expense of overall clicks) Its inspired by something I hacked together for my own tests.

I'm looking for feedback on 1) whether this is something desired for the gem 2) if so, I think it could be integrated into the ab_test method OR should be in its own helper.

[andrew]

Thanks @daviddening, it's been a long while since I really thought about the internals of this project but that seems like a useful feature, did you see any existing issues that are related to it?

I'd err on the side of putting it in it's own helper method rather than overloading ab_test.

Also all of the raise calls in a row feel a bit nasty, perhaps some of them could be pulled out into a single check method?

[daviddening]

@andrew Thanks! I did a quick refactor to bring the pull request in line with the suggestions.

This issue #275 seems closely related to the feature I built. I have also seen this feature requested on the StackOverflow.

I feel like I should add few more README updates as well, which I'll try to push up sometime tomorrow.

[daviddening]

@andrew Any further comments or thoughts on this?