allowed custom reporter for saucelabs/selenium

Hi,

This PR allows to specify a custom reporter for saucelabs or selenium when concurrency == 1.
I added an undocumented feature that allows to still use a custom reporter when concurrency > 2 but that requires a grunt-mocha-webdriver custom reporter that "buffers" test run outputs. See an example of custom reporter at https://github.com/saadtazi/gmwd-teamcity-reporter (similar to mocha-sauce-reporter).

Also, I added a `tunneled`option to skip the tunnel creation: can be useful if the tested site/webapp is public.

I know, I should have split the 2 features into 2 PRs... :)

Fixes #54.