More UI redesiging

A few fixes and enhancements, some more redesigning : 
* UI for password strength, using mattt's [Navajo](https://github.com/mattt/Navajo "Navajo"). Right now displays standard levels (@[invalid, weak, Reasonable,strong, very strong]), with a coloured progress view underneath
* Idea for a label displaying entropy of password. Right now display Shannon entropy, but the value will need to be adapted for usage of HKDF on the pw
* Disable some buttons if invalid fields (ex : code verification)
* Clear some text label's if user backtracks (ex : code verification)
* Redesign of group setup view
* Redesign password prompt view, with a shake animation on the padlock if password invalid
* Fixing some warnings, improper casts and small details like that

Here are some screenshots of the ui : 
Password view
![ios simulator screen shot 17 apr 2014 16 15 06](https://cloud.githubusercontent.com/assets/5308013/2732584/6534736a-c63c-11e3-9d28-3ade660b225e.png)

Password strength (with entropy label)
![ios simulator screen shot 17 apr 2014 16 16 12](https://cloud.githubusercontent.com/assets/5308013/2732600/8e816aca-c63c-11e3-8eba-3f2ee9477d2c.png)
![ios simulator screen shot 17 apr 2014 16 16 16](https://cloud.githubusercontent.com/assets/5308013/2732601/8e818a50-c63c-11e3-8d39-ce4460a24c13.png)
![ios simulator screen shot 17 apr 2014 16 16 14](https://cloud.githubusercontent.com/assets/5308013/2732603/8e81ceb6-c63c-11e3-8d96-ee632eb890d3.png)
![ios simulator screen shot 17 apr 2014 16 16 18](https://cloud.githubusercontent.com/assets/5308013/2732602/8e81bd90-c63c-11e3-8fb7-1d37df99114b.png)
![ios simulator screen shot 17 apr 2014 16 16 25](https://cloud.githubusercontent.com/assets/5308013/2732604/8e837d24-c63c-11e3-9619-11f1253792b8.png)
![ios simulator screen shot 17 apr 2014 16 16 20](https://cloud.githubusercontent.com/assets/5308013/2732605/8e8395f2-c63c-11e3-9741-e050f15c7b37.png)

Password strength without entropy label for comparison
![ios simulator screen shot 16 apr 2014 11 17 41](https://cloud.githubusercontent.com/assets/5308013/2732620/c9956846-c63c-11e3-833b-23106de9ba4c.png)

Group Setup View Controller
![screen shot 2014-04-17 at 16 29 26](https://cloud.githubusercontent.com/assets/5308013/2732625/db6a4c1c-c63c-11e3-85e4-80c37773b764.png)