Fix build and test tasks on Windows

This PR addresses two issues that I encountered setting up a development environment for Vimium on Windows:

Getting `cake build` to work:

1. Node.js's `child_process` module has difficulty spawning .cmd scripts, which is how the Coffeescript compiler runs on Windows machines.

Getting `cake test` to work:

2. The old method used by `dom_tests.coffee` to parse the current working directory is broken if a relative path with forward separators ('/') is passed to it. The `path` module works with either.