Add support for running multiple clusters on one machine 

This PR will allow for running multiple serf agents of different bind/rpc addresses. I've updated the README with explanation of changes. 

This will not require any changes from users of the cookbook who are not running multiple clusters on one box. 
Init.d script is mostly the same - I've had some trouble with how it currently figures out whether serf agent is already running, but eventually got around it by creating symlinks to serf binary for each agent (only if multiple agents are specified)

You can verify by: 
```
kitchen converge multiagent-ubuntu-1204
kitchen login multiagent-ubuntu-1204
```

then:

```
vagrant@multiagent-ubuntu-1204:~$ serf members -rpc-addr="127.0.0.1:7373"
first_agent    10.0.2.15:7946    alive
vagrant@multiagent-ubuntu-1204:~$ serf members -rpc-addr="127.0.0.1:7372"
second_agent    10.0.2.15:7945    alive
vagrant@multiagent-ubuntu-1204:~$ serf members -rpc-addr="127.0.0.1:7371"
third_agent    10.0.2.15:7944    alive
```

joining two agents:

```
vagrant@multiagent-ubuntu-1204:~$ serf join "0.0.0.0:7945"  -rpc-addr="127.0.0.1:7373"
Successfully joined cluster by contacting 1 nodes.
vagrant@multiagent-ubuntu-1204:~$ serf members -rpc-addr="127.0.0.1:7373"
first_agent    10.0.2.15:7946    alive
second_agent    10.0.2.15:7945    alive
```

Thanks for writing the cookbook!