first steps to new architecture

[lalomartins]

• 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 as well, you want to see it.

[arunoda]

As a general comment, try to put all these logic into a new module inside lib folder.