Spike client side fetch js source

An initial spike on an idea for displaying source code within error/failure output.

![screen shot 2014-08-13 at 22 01 46](https://cloud.githubusercontent.com/assets/104158/3905647/65218b64-22ea-11e4-8bd9-6d4bc9ba9867.png)

![screen shot 2014-08-14 at 08 18 18](https://cloud.githubusercontent.com/assets/104158/3914025/2f5ee4c6-2340-11e4-8bdb-eda896272e45.png)

Some of my rough thoughts on this idea/PR follow:

This class should end up being selected based on some kind of heuristic so that it's just a wrapper for whichever format is being used by the current browser.

It should also have a collection of `StackEntry` objects, each of which will then provide the `url`, `lineNumber` and `character`. This would then be a good place to insert `filename` and `sourceMapping`.