Support additional test runners via TestPlugin

[leggetter]

As covered in the Model & Plugin Design and to facilitate #7 it would be great to support additional TestPlugin implementations.

• - create the ability for TestPlugins
• - change the current test command so it uses the plugins
• - create a WebDriver test plugin - create a test plugin design that could theoretically support both js-test-driver and webdriver, though for now we won't support webdriver #87
• - ensure the WebDriver plugin works with any browser and with absolute paths - WebDriver doesn't support absolute paths to browsers #628
• - create a JSTD test plugin - convert the webdriver test command into a test plugin #85

[andy-berry-dev]

#880 (comment) has some info relating to this.