Added "goto test" and "goto implementation"

Fix ensime/ensime-server#428

With the cursor inside a class or object body, `C-c t` goes to a classes'  test (creating a stub if necessary) ; `C-c i` goes from test to implementation class.

I implemented the whole thing on the client side, because users may need to configure the behavior for their projects. I'm leaving the ensime-specific settings as an illustration : ensime test classes do not follow the most common pattern  (which is to put a test classe in the same package as the implementation class)

It would be great if people could try it out on various projects an give me feedback.