make sure that Kamon can be started/stopped in the same process

[ivantopo]

Currently, once you stop Kamon it gets into a weird state that doesn't allow you to start it again, and it is entirely necessary to be able to start it again if you want it to run in Play! applications in development mode.

As a side note, I want a full solution to #395, but the scope of that issue is kind of big, the idea here is to provide a solution that works for with the current way Kamon works.

[ivantopo]

This one wont fix on 0.6 and doesn't apply to 1.0, although we will need to fix this one instead: #502