basic logfmt

This is a rough implementation of logfmt output.  An example:

```
sample=true site_id=http-bit.ly location=undefined
t=2014-09-27T12:59:22Z exit_status=0 http_status=301 total_time=3.251284
name_lookup_time=3.125494 connect_time=3.177872
start_transfer_time=3.251259 ip=69.58.188.39 size=0
```