Make a Block Button at the top of the Palette

This is mostly in response to #375.

See the screenshots. It also makes it so that when you click make a block, the currently selected category is the default one. This currently only happens for the button though, and not from the contextual menu. 

I'd also like someone to look at the code for the shadows. I'm not sure they're correct. Actually, I'm not entirely sure the button _needs_ to be colored, but I thought it might be nice.

![screen shot 2014-04-04 at 1 21 05 am](https://cloud.githubusercontent.com/assets/1505907/2613357/2788c118-bbd5-11e3-9460-efe24d24f55a.png)
![screen shot 2014-04-04 at 1 21 29 am](https://cloud.githubusercontent.com/assets/1505907/2613358/27899b92-bbd5-11e3-8fcf-2524160c2961.png)
![screen shot 2014-04-04 at 1 21 19 am](https://cloud.githubusercontent.com/assets/1505907/2613359/278c8410-bbd5-11e3-9ca6-7b5b695e37fa.png)
![screen shot 2014-04-04 at 1 20 56 am](https://cloud.githubusercontent.com/assets/1505907/2613360/278d2ad2-bbd5-11e3-8e09-a26df3b97e4b.png)