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)

For now, please protect this pre-loading code with:
  if (config.level.edit_blocks) { ... }
So we don't pre-load the images on existing turtle levels.