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

briar-api/src/org/briarproject/api/event/IntroductionAbortedEvent.java

@@ -0,0 +1,23 @@
+package org.briarproject.api.event;
+
+import org.briarproject.api.contact.ContactId;
+import org.briarproject.api.introduction.SessionId;
+
+public class IntroductionAbortedEvent extends Event {
+
+	private final ContactId contactId;
+	private final SessionId sessionId;
+
+	public IntroductionAbortedEvent(ContactId contactId, SessionId sessionId) {
+		this.contactId = contactId;
+		this.sessionId = sessionId;
+	}
+
+	public ContactId getContactId() {
+		return contactId;
+	}
+
+	public SessionId getSessionId() {
+		return sessionId;
+	}
+}