Visual revamping the whole app

Made a huge visual revamp to the application. Created separate layout files instead of hard coding all the application design. Based on the Google Now interface, now each subject is displayed in a "card". Using a light theme as well.

When you try to edit or add a subject, a dialog will be shown instead of opening a new activity. A confirmation dialog will also be shown when trying to delete a subject.

Also fixed some backward compatibility bugs (that may have actually only appeared after the visual revamp).

If you want to see it in action, come see me, I've installed it on my N4 :)

![screenshot_2013-05-11-20-01-15](https://f.cloud.github.com/assets/1613383/492238/e6a95380-ba8e-11e2-9e8b-a9a9ff8d61fa.png)
![screenshot_2013-05-11-20-01-25](https://f.cloud.github.com/assets/1613383/492239/e6c48380-ba8e-11e2-98dc-c9c8b04d7bf7.png)
![screenshot_2013-05-11-20-01-36](https://f.cloud.github.com/assets/1613383/492240/e6dff980-ba8e-11e2-8c80-457d1ec6a54c.png)