New methods : rmdir, stat, lstat + callback support for get, put, putfo method

Hello,
I needed a few more methods for another project of mine, so I ended up adding them directly.

rmdir, stat and lstat are self-explanatory.

I also added callback support for get, put and putfo method. Use cases are :
- update the status of the transfer with the amount currently transferred
- interrupt the transfer by raising an exception in the callback method