Add block for setting line style/pattern

This submission covers addition of the set line style/pattern block. The solid line within the block will return the canvas to using default line drawing behavior. Currently, setting a color after setting a pattern will also return the user to default line drawing behavior.

![screenshot](https://cloud.githubusercontent.com/assets/6125532/4654384/a4350206-54b6-11e4-9bf4-16e70814097e.png)

Seems like we do this enough that it might be nice to just have a Turtle.setPattern which takes either an image (then sets isDrawingWithPattern to true) or null, (in which case it sets patternForPaths to a new Image and sets our boolean to false.