Extract contact exchange protocol from BT introduction protocol

...org/briarproject/api/contact/ContactExchangeListener.java

...src/org/briarproject/api/contact/ContactExchangeTask.java

...-api/src/org/briarproject/api/crypto/CryptoComponent.java

...src/org/briarproject/contact/ContactExchangeTaskImpl.java

briar-core/src/org/briarproject/contact/ContactModule.java

...core/src/org/briarproject/crypto/CryptoComponentImpl.java

...-core/src/org/briarproject/invitation/AliceConnector.java

briar-core/src/org/briarproject/invitation/BobConnector.java