Add bootstrap style to devise sign in view as example

#### Bootstrap example
I've added a couple of bootstrap classes as an example in the ```sign_in view```. 


#### More about bootstrap
The documentation is pretty extensive and it's quite good, it can be found [here](http://getbootstrap.com/css/). Related with the layout of the app there are plenty of pages with bootstrap layout examples (ie. [this](http://www.bootply.com/templates)). 
In the ```forms``` section of the documentation you can find all the classes you could add to the form element in order to style them. Normally, form elements are wrapped in a ```form-group```.