Add simple command-based execdriver.

This was done pretty quickly (and is my first docker patch), so it may need refinement and definitely needs a bit more testing prior to merging. I'm sending the PR right away in order to get feedback!

These patches do two things:
1) Plumb through options (a simple string) to the exec driver. This may be necessary for future drivers (perhaps even slightly more complex).
2) Enable a very simple backend that will allows one to specify a command to run for initialization (on NewDriver) and for starting docker instances. In the long run, I think this will be very valuable in building out new execdrivers and enables writing execdrivers as plugins (by writing a wrapper, then using the shell execdriver).