feat(plug-in): use per file stubs / watchify only

This commit rewrites the inner workings on the plug-in. The major
changes are:

  * it relies soley on `watchify` for watching file bundle changes
    during autoWatch=true

  * it creates a single browserified bundle + stubs for each
    test file

  * it improves the karma config file syntax (got rid of the
    /**/*.browserify preprocessor hack, use config.files for
    test files in addition to preprocessor: pattern: [ 'browserify' ])

  * it waits for initial bundle build before running any test cases