Separated key agreement algorithm from signature algorithm.

api/net/sf/briar/api/crypto/CryptoComponent.java

components/net/sf/briar/crypto/CryptoComponentImpl.java

components/net/sf/briar/plugins/InvitationStarterImpl.java

components/net/sf/briar/protocol/UnverifiedBatchImpl.java

test/net/sf/briar/ProtocolIntegrationTest.java

test/net/sf/briar/plugins/InvitationStarterImplTest.java

test/net/sf/briar/protocol/ConstantsTest.java

test/net/sf/briar/protocol/UnverifiedBatchImplTest.java