Redesign Signup views

* New design for sign up views, based on RMStepsController.
* Color palette closer to Droid + UIColor category
* Fix country prefix in some cases appeared with garbage values
* Responsive design : iphone4/4s vs iphone 5


Note : Fixed the fact that the 'Send' was clickable if the umber was invalid.
Some screenshots : 
![ios simulator screen shot 16 apr 2014 01 03 05](https://cloud.githubusercontent.com/assets/5308013/2714186/4016a15c-c4f2-11e3-95cd-4fb7587d2935.png)
![ios simulator screen shot 16 apr 2014 01 03 19](https://cloud.githubusercontent.com/assets/5308013/2714187/4016e41e-c4f2-11e3-8eab-53f907009a41.png)
![ios simulator screen shot 16 apr 2014 01 03 24](https://cloud.githubusercontent.com/assets/5308013/2714188/4016f35a-c4f2-11e3-967a-9e23b40421ab.png)
![ios simulator screen shot 16 apr 2014 01 03 32](https://cloud.githubusercontent.com/assets/5308013/2714189/4017d310-c4f2-11e3-8fed-a5e7b3faf135.png)
![ios simulator screen shot 16 apr 2014 01 03 34](https://cloud.githubusercontent.com/assets/5308013/2714190/401855ce-c4f2-11e3-9753-d0df1db2a3c5.png)
![ios simulator screen shot 16 apr 2014 01 03 37](https://cloud.githubusercontent.com/assets/5308013/2714191/40188364-c4f2-11e3-96b3-f7ebaf9c0a14.png)
![ios simulator screen shot 16 apr 2014 01 03 45](https://cloud.githubusercontent.com/assets/5308013/2714192/402ae950-c4f2-11e3-8c72-997a5c47cc70.png)