Integration Tests for Introduction Client

* normal session where both introducees accept
* normal session where the first introducee declines
* normal session where the second introducee declines
* one session where a contact is introduced to herself
* one session where two identities of the same contact
  are introduced to each other

This introduces a new IntroductionAbortedEvent to signal when the
protocol was aborted. It is not yet used in the UI.

It closes #276