Re-enables buttons and links after going back

Firefox has a weird behavior where when you hit the back button it keeps
the submit buttons and links on disabled state which even reloading the
page do not re-enables them.

Using the pageshow event, which is triggered every time a page loads, we
can keep the rails-ujs behavior consistent with all other browsers.

See https://developer.mozilla.org/en-US/docs/Using_Firefox_1.5_caching

[Fixes #357]
Merge pull request #385 from sobrinho/master

Re-enables buttons and links after going back