highlight the hovered and clicked nodes an their ancestors

I added some background highlighting to keep track of where is everything in big JSONs.
It gives an increasingly darker background as it goes deeper. Blue for the hovered node:
![jsonview-hover](https://cloud.githubusercontent.com/assets/1610221/3239556/d9a426da-f10e-11e3-85cc-5b3f4cde0807.png)
And green for the clicked node (stays highlighted until you click somewhere else so you can scroll and keep track of it):
![jsonview-clicked-and-hover](https://cloud.githubusercontent.com/assets/1610221/3239555/d9a37a1e-f10e-11e3-8e98-4234a317dcf3.png)