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)

On 7/20/14 2:11 AM, Jens Mönig wrote:
> * rename the block to 'print'
> * use 'px' instead of 'pt'
> * get rid of the 'size' parameter and take the pen size attribute instead
> * only actually render the text if the pen is "down", otherwise just move as in line 2785
> * make sure not to draw  the default line in line 2785 if you're already printing text

Sorry I slept through this interesting discussion.

The most important thing to me is to have the block at all; then I can always 
write a custom block that works the way I want.

But now that we've agreed on a monospace font, my preferred behavior would be 
for the sprite not to move at all.  We only introduced the idea of a moving 
sprite as a kludge to avoid the need for a pixel-width-of-text block.  Yes, I'm 
thinking about labelling a graph, and I want to control the sprite motion so 
that the labels line up with the tick marks on the axes.  But I can save and 
restore the sprite's position, so it's not crucial.

About points vs. pixels and pen size vs. separate size, the reason I suggested 
keeping point size as an input in the block was merely to have a sensible 
default, as opposed to a one-pixel default.  But making Jens happy is more 
important to me than winning on this issue.