Label block

This replaces #362  and makes the text block a very simple label text block. 

It takes in a string, and a font size (in point values). 

It will:
* Rotate text with the sprite
* Move text to the end of the word
* Draws text at alphabetic baseline
* Always stamp LTR. There will need to be a bit of work done for RTL.

Example:
![screen shot 2014-06-29 at 10 22 34 am](https://cloud.githubusercontent.com/assets/1505907/3423921/a02170cc-ffb2-11e3-9d27-0bd111a033d5.png)

@brianharvey For this one....

He and I had a discussion about this in his office a while back. I think the idea was that alphabetic would be closer to how you'd expect things to be written. I personally wouldn't except to much text to be drawn precisely with this, at least if support for 'text box' type costumes are supposed to be supported some time. 

As far as pt vs px, I suppose it doesn't matter tons, but I explicitly chose pt values because they are what I would expect students to be familiar with in terms of font sizes (i.e. from word processing). Px is also sometimes less consistent across devices with different pixel densities.