Expose the encryption and authentication overhead without breaking

encapsulation.

This should allow callers to calculate maximum packet sizes without
knowing the details of the transport protocol.

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

@@ -19,4 +19,8 @@ class NullConnectionEncrypter implements ConnectionEncrypter {
 	public void writeMac(byte[] mac) throws IOException {
 		out.write(mac);
 	}
+
+	public long getCapacity(long capacity) {
+		return capacity;
+	}
 }