Fix #204 - add support for XHTML

Together with colors-xhtml in the candy-plugins repository (branch
feature/xhtml-colors) this will enable Candy to show validated
XHTML if enabled with the `enableXHTML` config flag.

In order to make this work, I had to fix the muc plugin of strophe.
Until strophe/strophejs-plugins#14 is not merged, this is done manually.
Remove unneeded comment
Make XHTML parsing more robust & display error in case there is one
Fix XHTML parsing in text

- Escape textNode html tags (they're not allowed there)
- Remove margin from p-Tags in messages (native clients tend to use it
  and it crashes the layout a bit)
- Escape submitted text before sending it
Revert escaping on submit (instead escape in plugin)
Merge dev
Merge branch 'dev' into feature/xhtml-colors
Fix #220 invalid html tags generated

Because the createHtml used previously the XMLGenerator
of Strophe, it assumed <br> to be a real tag (<br></br>),
therefore it didn't work out.

Switched now to some jQuery stuff (not beautiful but it works).
Merge dev
Merge dev
Merge branch 'dev' into feature/xhtml-colors
Merge branch 'feature/xhtml-colors' into dev

Fixes #204
Update strophejs-plugins
Upgrade Strophe.js version
Fix sending of messages due to muc plugin upgrade