feat(windows compatibility): correct paths in Windows

Please note: I've got a PR in to square/es6-module-transpiler: https://github.com/square/es6-module-transpiler/pull/16 with similar changes for running under Windows.  Together with that PR, I'm able to build and run the tests for grunt-es6-module-transpiler.

A couple of changes:
- replace backslashes from native format paths before using them in AMD require/define situations
- normalize linefeeds and file paths in tests