Incremental download/upload

[alexist]

We have some large repositories to copy from one repo to another repo. When the process is interupted (network issue for example), we have to restart from scratch.

It would be great to allow incremental transfert with an incremental option :

When the incremental flag is set to true, wagon will store artifacts in a local folder. If the transfert is interrupted, we can restart the process and wagon will not download file already present in local folder.
And when file are already present in target in target repository, wagon will not upload theses files

#38