Server-side progress hooks

[superadm1n]

I would like to have the listen method in the TFTPServer class split up into smaller logical methods so its possible to override them and get in the middle of the "listening" process allowing for print statements etc. I currently am trying to solve the problem of printing to the console in a CLI application when a client connects to the server. I did fork the repo and have made a few changes to allow for this and I can commit the code and submit a pull request for this change or if there is a better way to achieve this I would love to hear how to.

[msoulier]

I put in a packethook in the client download method, but there does not appear to be anything on the server side. This would be a good addition.

[msoulier]

Also note that issue #32 specifically requested a callback on completion.

[superadm1n]

I did take a stab at the implementation of this, take a look and let me know what you think.