Support Parallel_tests 0.16.5, arbitrary parallel_tests options

Added support for parallel_tests 0.16.5.

Can now pass arbitrary options to parallel_tests from the Sauce rake
tasks, using the 'parallel_test_options' argument, eg:

`bundle exec rake sauce:features parallel_test_options='--group-by
scenarios'

Made browser_for_file into browsers_for_location, taking filename and
location in file.

Added tests for rake parallel tasks, browsers_for_location