Implement SessionEncoder and SessionParser

...n/java/org/briarproject/briar/api/introduction2/Role.java

...g/briarproject/briar/introduction2/IntroduceeSession.java

...org/briarproject/briar/introduction2/IntroduceeState.java

...g/briarproject/briar/introduction2/IntroducerSession.java

...org/briarproject/briar/introduction2/IntroducerState.java

...iarproject/briar/introduction2/IntroductionConstants.java

...ava/org/briarproject/briar/introduction2/PeerSession.java

...in/java/org/briarproject/briar/introduction2/Session.java

.../org/briarproject/briar/introduction2/SessionEncoder.java

.../briarproject/briar/introduction2/SessionEncoderImpl.java

...a/org/briarproject/briar/introduction2/SessionParser.java

...g/briarproject/briar/introduction2/SessionParserImpl.java

...main/java/org/briarproject/briar/introduction2/State.java

...ar/introduction2/SessionEncoderParserIntegrationTest.java

...riarproject/briar/test/BriarIntegrationTestComponent.java