Show progress information for file transfer

See #62.

For example:
```
Get a remote file (/tmp/fixture-9260-X) to local (/home/...)
Starting transfer 268,435,456 bytes.
Transferred 6% in 3.004 secs.
Transferred 36% in 6.117 secs.
Transferred 70% in 9.117 secs.
Finished transfer 268,435,456 bytes (23,245 kB/s). Took 11.549 secs.
```