Add support for testing on BrowserStack

[lddubeau]

This means updating wed's selenium_config.py and selenic.

1. Add a REMOTE_SERVICE configuration option to select which of SL or BS is going to be used.
2. Make REMOTE_SERVICE overridable by passing a -D to behave. (Pass the -D option to the Builder's options argument, then in selenium_config.py check for the presence of that argument and override if present.)
3. Have a BROWSERSTACK_CREDENTIALS analogue to SAUCELABS_CREDENTIALS.
4. Review how the capabilities are set in selenium_config.py and in selenic's code so that anything that is specific to SL is adapted for BS use.
5. Create a tunnel appropriate for BS. (Just like we do with sauce connect.)
6. Add a way to set the final test status through the API.

[lddubeau]

This has been implemented.