Windows fix regarding paths

I tested it on my Win 8.1
None of sample projects worked correctly on my PC. When I started digging I found that a lot of code (both Scala and JS) relies on / path separators whilst windows gives \\. And pull https://github.com/sbt/sbt-rjs/pull/17 just fixed error message but not the problem.

My solution is to replace \ with /. Windows understands such paths too and other code can just rely on one path separator.

I've done replacement where was needed. Few paths in app.build.js are still generated with backslashes, but as far as I tested they don't break anything.