Fixed: OPDS XML broken because of comment html

Calibre book->comments contains html. But XML doesn't allow any
unclosed tags (e.g. <br>).
Solution: escape tags as htmlentities.
Changed: Kindle button UI

- Send-To-Kindle button is now grouped with the download buttons.
Button click opens a jquery mobile popup.
- unified error message is displayed to user if ajax call returns with
error.
Merge pull request #44 from janeczku/master

Kindle button UI changed / OPDS fixed