Made secret keys erasable from memory.

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

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

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

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

...nents/net/sf/briar/transport/ConnectionDecrypterImpl.java

...nents/net/sf/briar/transport/ConnectionEncrypterImpl.java

...s/net/sf/briar/transport/ConnectionReaderFactoryImpl.java

components/net/sf/briar/transport/ConnectionReaderImpl.java

...ents/net/sf/briar/transport/ConnectionRecogniserImpl.java

...s/net/sf/briar/transport/ConnectionWriterFactoryImpl.java

components/net/sf/briar/transport/ConnectionWriterImpl.java

...onents/net/sf/briar/transport/PaddedConnectionWriter.java

test/net/sf/briar/transport/ConnectionDecrypterImplTest.java

test/net/sf/briar/transport/ConnectionEncrypterImplTest.java

.../net/sf/briar/transport/ConnectionRecogniserImplTest.java

test/net/sf/briar/transport/FrameReadWriteTest.java

test/net/sf/briar/transport/TransportTest.java