Give envconsul some TLC ;)

This changes envconsul to use Consul Template's libraries for polling and adds much more test coverage. There are still some discussion points:

- `-reload` is deprecated and confuses me. If you specify `-once`, envconsul will query exactly once and run the command exactly once. It will never trigger for new changes, so it will never fire restart.
- `-terminate` was never actually released, but I don't understand the point. It seems very backwards to me... when activated, the child process is killed if any keys change (#12). I really cannot find a use case where you would want to kill a process because a key changed.
- `errExit` is deprecated. If you specify `-once` and Consul is down, the entire process will fail. If the process is already running, it will continue to run until Consul comes back online.

What do you think? I generally prefer simpler UX and less command line options. I don't think any of those command line options were intuitive, and I'm still uncertain what some of them do :smile:.

As bonus points:

- Added quiescence :smile: 
- Added the skeleton work for watching multiple key prefixes

/cc @armon @mitchellh @ryanuber