Enhance responsive UI for the login screen.

Enhance responsive UI for the login screen. This especially makes things
nicer for mobile. It actually makes some nice subtle adjustments for all
devices, too.

For mobile, you can see the content is properly centered with these changes.

Mobile device current:
![orig](https://f.cloud.github.com/assets/2394772/1981163/4b60351c-83d1-11e3-9cad-d5d0a7c0f6b7.PNG)

Mobile device new:
![new](https://f.cloud.github.com/assets/2394772/1981168/5a189cb6-83d1-11e3-8814-e3d7c441ea70.PNG)

For medium to large devices you can see a slight increase in the width of the login fields. This is because I removed the 20px padding. It doesn't make a big difference for medium and large devices but it solves the problem for smaller devices. Also, it has the nice side effect of correcting the word wrapped 'Send again' text. It also lines up the right side of the fields and the 'Forgot your password?' button.

Med/Large device current:
![med_orig](https://f.cloud.github.com/assets/2394772/1981153/232fafc8-83d1-11e3-86c9-43dfe06f6d95.png)

Med/Large device new:
![med_new](https://f.cloud.github.com/assets/2394772/1981154/26c789f8-83d1-11e3-9d71-5c0f4fb0fc08.png)