Clean up the monitor command.

The monitor command now:

* Has more pythonic code
* Limits precision of progress elements (12.3% instead of 12.347812...%)
* Uses a carriage return to have print progress replace the previous
  progress line.

For example:

Monitoring printer, use ^C to interrupt.
Updating values every 5.000000 seconds.
Print progress: 0.3%

Previously, the line "Print progress: 0.3%" was "Print progress: 0.2%",
etc.
Merge pull request #320 from colah/clean-monitor

Clean up the monitor command.