Added method to replace slashes with File.separator to make sure it'll run on any OS
Changed osAgnosticPathToExe initial value

As pointed out, this changes the initial value of osAgnosticPathToExe to
be the karmaExecutable value passed in so that if no executable is
specified in the pom then the plugin will use that.
Merge pull request #22 from PhilHardwick/OS_agnostic_karma_executable_path

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