Parse XML logs from FTP

[vladvelici]

The messages sent over STOMP are updates, and if some updates are missing the data does not reflect the reality.

Adding a way to parse the darwin logs from FPT given a start timestamp would help address this issue.

The messages taken from the logs need to be sent over to the listener implemented by the client (user of this library) just as the STOMP messages, and the order should be as they were sent from Darwin. This makes all the complicated logic of going to FTP, downloading XMLs, and then connecting to STOMP transparent to the client.

Maybe an extra, optional argument on the on_*_message methods to indicate the source would be nice.