Added a "grunt testserver" task for inspecting tests in a browser

Automated tests in a headless browser are a great, but sometimes I'd rather inspect and debug tests in a "real" browser. Perhaps I am not the only one ... Why not make life a little easier by adding a `grunt testserver` task?

It essentially is the same as `grunt server`, but 

- it sets the docroot to the test directory
- it merges the test and app directories

just like the test task does.

Cheers,

Michael