Generate an X-VCAP-Request-ID for each request and capture it in the access log

Based on a suggestion from @phanle, add vcap request id to the access log for improved correlation between response times and request activity in the cc log.

```
api.10.244.0.34.xip.io - [25/02/2014:14:09:19 +0000] "GET /bulk/apps?batch_size=500&bulk_token={} HTTP/1.1" 200 281 "-" "Go 1.1 package http" 172.16.118.134:37262 vcap_request:0b5599bf-1247-48ed-aaf0-212142b41203 response_time:0.008804337 app_id:
```