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)). 
There are different types of buttons, al of them share the ```btn``` class that sets the base stuff for them. If you want to change the color you should add a second class like ```btn-primary``` or ```btn-info```.
Also there are classes to change the buttons size like ```btn-lg```.