feat(command): add exec-watch command

This is a simple command that will run forever exec the command passed
into its argument every time there is a configuration change. This is
perfect for glue code that generates configuration files, kicks
services, etc.
fix(command): make the exec-watch prefix ETCD_WATCH

this was suggested by mike and makes a lot of sense. Fix
feat(command): remove --index on watches
Merge pull request #26 from philips/exec-watch

feat(command): add exec-watch command