added replacement of blank spaces with %20 (tests pending)

I recently had the issue that we have a blank space in the path where our JMVC app is located. Unfortunately we cannot change that whitespace. The problem is that the build terminates with some Java uri format exception.

I traced the problem down to env.js which apparently does no encoding of the URI. The fix provided in this pull request seems to resolve the issue. Unfortunately I was not able to execute the automated tests (btw, could you include a guide on running them on the main readme.md of your project??). What I did, though, was to execute a couple of builds with the modified env.js on different projects in our codebase and it seemed to work properly.

Related links:

- http://forum.javascriptmvc.com/topic/build-js-on-windows-with-space-in-user-error