Fixes #459. ImplicitSender should not change when creating TestProbes

Fixes bug #459

- Added `NoImplicitSender` to `TestProbe` class to make it not set its ´TestActor´ as implicit sender
- Removed deprecated constructor from `TestProbe` to prepare for v0.7
- Added test that verifies that the bug has been fixed, and a test that creating new `TestActor`s do not change implicit sender.