npr/responsiveiframe#7

Added checks to ensure that xdomain is correct. I chose to throw errors instead of handling it gracefully because I think that the xdomain - origin mismatch should not result in running code. Better to fix the error than to breeze over the mismatch.

Also, I wasn't able to get grunt working. I think that the current setup is for grunt 0.2.x, but it isn't in the package.json. Let me know if I did something wrong ( this is my first time working with grunt). I may just file another issue, and we can handle it there.