some svg block validation

This change doesn't do nearly as much as I'd like, but I've already spent a lot more time on it that I intended/expected, and I think it's time to move on.

This is an attempt to do some validation of svg paths to make sure I'm not breaking things. It's a bunch of Javascript that goes through the blocks in the workspace and outputs the paths. In then has a means of exporting this, such that we can later output the paths for the same blocks and ensure that they haven't changed.

I'd started out trying to do this in the node environment, but it didn't work because we call getComputedTextLength which isn't supported by our simulated canvas. I then moved it to cucumber, thinking the transition would be easy.

One problem is that different browsers display the svgs slightly differently. The validation I have here passes when I run locally, but fails on all of our browsers. I didn't look in detail, but Chrome33Win7, for example, was just off by 2.5 pixels in one spot. As such, I made this scenario local_only, meaning it shouldn't run unless you're running the local chromedriver. This means it's only going to get coverage when run manually.

Right now I'm just validating the artist blocks from http://learn.code.org/s/1/level/108. It should be easy to add others.