feat text-fields: fixed label and valid input state

There's now a new feature for text fields : fixed label (contrary to floating label) and a valid input state. 
The text fields demo has been update.

Here is the valid state :

![capture d ecran 2014-12-01 a 10 14 27](https://cloud.githubusercontent.com/assets/1220284/5243327/f9b4daac-7942-11e4-9997-5893407d42fe.png)
![capture d ecran 2014-12-01 a 10 14 48](https://cloud.githubusercontent.com/assets/1220284/5243329/ffbaec7a-7942-11e4-8a39-bc12a2c74eda.png)

And fixed labels :

![capture d ecran 2014-12-01 a 10 14 33](https://cloud.githubusercontent.com/assets/1220284/5243335/0f3bfa5e-7943-11e4-9dcb-e6b8ad1f18cb.png)
![capture d ecran 2014-12-01 a 10 14 57](https://cloud.githubusercontent.com/assets/1220284/5243341/157b90b4-7943-11e4-91fd-fa6febabe97e.png)

`isDefined` isn't sufficient. If the user use `fixed-label="false"`, `isDefined` will be true even though the user won't want this options.
And you have to watch the value, not just set it one time in the link.