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()`...

I also noticed when debugging this issue that we broke Mobify post-flood (post document.write). Not good! This was determined to be broken during the merge of the swift feature branch. This has been fixed, and a regression test has been added.

@noahadams could you review plz?