Transfer files first to a temporary filename

[pnuu]

The files should be transferred to a temporary filenames (to a temporary directory?) and then moved to the proper filename/location. This would help Posttroll agnostic scripts working with the files, so that they are not taken to use prematurely before the transfers have completed.

See also #91

[pnuu]

This would need to be somehow configurable. If the Client doesn't have access to the the transferred file (e.g. an external ftp server or scp/ssh location) it can't rename the file.