Indicate password strength using zxcvbn

There are still a few problems:

- [ ] Load the script asynchronous (how can I get the path of the 3rdparty folder in JS?)
- [ ] Make the UI nicer - I'd like [such a cool bar (scoll-down)](https://tech.dropbox.com/2012/04/zxcvbn-realistic-password-strength-estimation/) and when clicking on the (i) there should more info displayed (time to crack, entropy etc...) @jancborchardt Interessted?

![u8jh6nhwnjpa](https://f.cloud.github.com/assets/878997/253174/b8144cb0-8bc6-11e2-9fc6-93b13f2fde1e.png)


- [ ] Clean up the JS / HTML (maybe even make it a jQuery plugin so it's reusable) - is here any jQuery guru who wants to do this? @Raydiation @raghunayyar? *hides* ;-)

Requires https://github.com/owncloud/3rdparty/pull/20

Ref #2048