Add ePub content extraction and handling

TODO: Since we retrieve reading content as an input from the end user, need
to add some kind of input size limiting to not accidentally run into DOS. In
particular, big ePub archives can effectively kill the performance of all
currently running sessions.
Fix spin buttons on wpm in Firefox v29.0.1

I have no clue why those spin buttons suddenly became
visible in new version of Firefox (29.0.1)