Change to the process builder to replace slashes with file.separator

I found that when using a local install of karma (i.e. without -g in the npm install) specifying a relative path to the executable would not work across both linux and windows systems. The ability to use a local install of karma-cli in node_modules/.bin means the project can keep it's dependencies localised, removing the need for karma to be installed globally.

The extra method just does a simple replace on '/' and '\' with File.separator.

I have also added a bit to the documentation comments on the properties to make it a bit clearer what the reporting properties do.

There's a possibilty to make the default value of junitReporterFile the same as the value of junitReporter.outputFile in the karma.conf.js file but that would require a JSON library being added to extract that value. I haven't done this here but if it seems reasonable to do then I can add this in another pull request.