first steps to new architecture

* load plugins in package.json for each test directory
* install any plugins not found
* call `instrumentClient()` and `instrumentServer()` if defined

I was able to run my webdriver-based tests using this version of Laika (and removing the lines in the test code that manually import laika-webdriver and wrap the client, of course). I pushed the modified [laika-webdriver](https://github.com/lalomartins/laika-webdriver) as well, you want to see it.