Moved plugin API from transport package to plugins package and merged

api/net/sf/briar/api/plugins/BatchTransportCallback.java

api/net/sf/briar/api/plugins/BatchTransportPlugin.java

api/net/sf/briar/api/plugins/StreamTransportCallback.java

api/net/sf/briar/api/plugins/StreamTransportPlugin.java

api/net/sf/briar/api/plugins/TransportPlugin.java

api/net/sf/briar/api/transport/BatchTransportReader.java

api/net/sf/briar/api/transport/BatchTransportWriter.java

...net/sf/briar/api/transport/StreamTransportConnection.java

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

...nents/net/sf/briar/plugins/bluetooth/BluetoothPlugin.java

...briar/plugins/bluetooth/BluetoothTransportConnection.java

components/net/sf/briar/plugins/file/FilePlugin.java

...onents/net/sf/briar/plugins/file/FileTransportReader.java

...onents/net/sf/briar/plugins/file/FileTransportWriter.java

...nents/net/sf/briar/plugins/file/RemovableDrivePlugin.java

...nents/net/sf/briar/plugins/socket/SimpleSocketPlugin.java

components/net/sf/briar/plugins/socket/SocketPlugin.java

...et/sf/briar/plugins/socket/SocketTransportConnection.java

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

test/net/sf/briar/plugins/bluetooth/BluetoothClientTest.java