Adding run_once functionality

[hdahme]

Adds the ability to lock a command to the calling process - with the optional limit that the command can only be run once an interval

Usage

To run the echo command once, across a cluster of servers, and to ensure that it's only run once every 60 seconds

consulate run_once -i 60 echo_hello 'echo hello'
>>> hello
consulate run_once -i 60 echo_hello 'echo hello'
>>> Last run happened fewer than 60 second ago. Exiting

This relies on using consulate's existing acquire_lock function as well as setting a lock with a unique name, such that subsequent executions can reference the last time the command was run

[hdahme]

Closing #43 in favour of this

[gmr]

Looks good, I'll start prepping a new version soon so we can get this released.