CF_TRACE option for cf logs

This is for https://github.com/cloudfoundry/cli/issues/214

CF_TRACE now shows loggregator websocket connection handshake

Sample output:

```
$ CF_TRACE=true out/cf logs dora

...

WEBSOCKET REQUEST: [2014-08-11T15:50:20-06:00]
GET /tail/?app=55992983-8d1a-492b-bdcf-0ea3369eebfa HTTP/1.1
Host: wss://loggregator.10.244.0.34.xip.io:443
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Version: 13
Sec-WebSocket-Key: [HIDDEN]
Origin: http://localhost
Authorization: [PRIVATE DATA HIDDEN]


WEBSOCKET RESPONSE: [2014-08-11T15:50:20-06:00]
HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-Websocket-Accept: kkI2BZQP6SWt5GRusyIENa91o7M=

Connected, tailing logs for app dora in org myorg / space myspace as admin...
```