Initial implementation of pausing

* logging of pid upon startup
* agent spawning can be toggled on or off via SIGUSR1
* state changes are logged

ref #4 

Example:

```
# terminal 1
$ haggar
2014/08/06 13:05:02 master: pid 28484

# terminal 2
$ kill -USR1 28484

# terminal 1
2014/08/06 13:05:26 mater: spawn_agents=true
```

Caveats:

If the master process is sleeping due to jitter, it won't pause and print status until after it wakes up.  I will fix in a subsequent PR.