V2.0 back button

Fixes back button issue for IE10 and Firefox raised by issue https://github.com/mobify/mobifyjs/issues/137.

Started writing a regression test, but was thwarted by the fact that the `history` object is shared between parent and iframes, thus I could not simulate `history.back()`...

Also fixes issue I noticed that Mobify object is not declared after the flood, and thus when trying to re-load the library post-flood, you get an error. Noticed because IE was much more vocal about the error then FF was.

@noahadams could you review plz?