Unique sessions, remote retries, pre and post commands, eeprom support

Added some things:

* A unique string is added to control paths for each execution and to the remote file names. This lets you run several avrdude-scp commands in parallel if you have multiple programmers on your remote.
* Added --retry option to do automatic retries on remote side without uploading the files again. Useful when the programmer is a bit unreliable and requires multiple attempts to get into programming mode.
* Added support for writing/reading/verifying the eeprom as well.
    (are there other AVR memories that should be supported ... are read/written to file?)
* Added support for executing commands before and after avrdude on the remote.
    (I did not remove your stty command, but made it arduino specific, but I actually think this is something that should be removed and be called by the user with the --pre option instead)
* Updated README to reflect additions.