Allow canceling download of large files

[dlech]

Spin off of #37.

We can currently cancel downloading many files by finishing the download of the current file and skipping the rest. However, it would be nice to be able to cancel the download of a large file in the middle of the download as well.

This will require changing the implementation of the Device.put function to be cancelable instead of relying on the ssh2 module fastPut. It will probably also require that we save the remote file with a different name and then rename/replace the file with the correct name so that we don't end up with half of a file on the remote device.

[dlech]

This should also be done for uploading files.