support for multiple agent installation, Serf LWRP

This PR will allow for running multiple serf agents of different bind/rpc addresses. I've updated the README with explanation of changes. It adds a Serf LWRP which is used by the updated default recipe.

This will not require any changes from users of the cookbook who are not running multiple clusters on one box. 

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
```

send an event to one agent, handled by a downloaded event handler:
```
serf event -rpc-addr="127.0.0.1:7371" rvm3
Event 'rvm3' dispatched! Coalescing enabled: true
vagrant@multiagent-ubuntu-1204:~$ cat /opt/serf/logs/agent_3.log
==> Starting Serf agent...
==> Starting Serf agent RPC...
==> Serf agent running!
         Node name: 'third_agent'
         Bind addr: '0.0.0.0:7944'
          RPC addr: '127.0.0.1:7371'
         Encrypted: false
          Snapshot: false
           Profile: lan

==> Log data will now stream in as it occurs:

    2014/06/10 23:52:27 [INFO] agent: Serf agent starting
    2014/06/10 23:52:27 [INFO] serf: EventMemberJoin: third_agent 10.0.2.15
    2014/06/10 23:52:28 [INFO] agent: Received event: member-join
    2014/06/10 23:56:36 [INFO] agent.ipc: Accepted client: 127.0.0.1:41617
    2014/06/11 00:06:06 [INFO] agent.ipc: Accepted client: 127.0.0.1:41619
    2014/06/11 00:06:06 [DEBUG] agent: Requesting user event send: rvm3. Coalesced: true. Payload: ""
    2014/06/11 00:06:07 [INFO] agent: Received event: user-event: rvm3
    2014/06/11 00:06:07 [DEBUG] agent: Event 'user' script output: mkdir: cannot create directory `//.rvm': Permission denied
    2014/06/11 00:06:07 [ERR] agent: Error invoking script '/opt/serf/event_handlers/agent_3/rvm-installer': exit status 1
```

Hopefully this is cleaner than my last attempt.
That'll be determined by agent_exists?, though. You can verify by changing named_agent in kitchen yaml, for example, and re-running converge.