Windows Fix - Paths could not be created

There were problems when generating temp files to serve the "istanbul-ised" scripts back to phantom.
When the C:/testfile.js was stored, it was attempted to save this at .../temp/C:/testfile.js which is not a valid path. This fix escapes the path to generate just a single file under temp.

If you except this pull, would you consider redeploying to npm?

Thanks, Florian Rüberg (florian.rueberg@gmail.com)