add floating punctuation toggler for EPUBs

This patch should also fix #651.

Before applying this patch:
![screenshot_2014-07-22-21-35-42](https://cloud.githubusercontent.com/assets/751535/3658868/6a2235f8-11a8-11e4-89e9-585386dab17e.png)

After applying this patch:
![screenshot_2014-07-22-21-33-32](https://cloud.githubusercontent.com/assets/751535/3658866/66c5d05e-11a8-11e4-885d-5e9542543607.png)