Use more reliable serial device naming in Windows.

* Strip leading "/dev/" from MONITOR_PORT before handing to avrdude in Windows.
* Use the more widely available awk tool instead of bc to subtract 1
    from COM ID (as opposed to `bc`).
* Allow Windows user to specify "com1" or just "1".
* Document MONITOR_PORT format for Windows users.
Merge pull request #155 from peplin/monitor-port-check

Use more reliable serial device naming in Windows.