Set up sourcemaps with WebStorm

- add WebStorm project config files (incl. AtScript watcher)
- Karma consumes files transpiled by the watcher (instead of using preprocessor)
Karma: external source maps loader

So that it can load source maps from external file.
This is used when the transpilation is done by WebStorm, outside of Karma.
WebStorm: wait for all watchers before running Karma

Temporary work-around for https://youtrack.jetbrains.com/issue/WEB-14104

This is not ideal fix:
- you can trigger unit tests before a watcher even starts,
- you need `gulp build` in the case some file is not transpiled yet.
Fix gulp-traceur to produce the same output as WebStorm watcher
WebStorm: generic sourceRoot for source maps
WebStorm: update the project