Added support for request headers

This is a fix for the issue I raised in #452. Basically I've added support for request headers, and fixed a minor bug with the user agent argument on one of the phantomjs calls

I added this because I'm testing APIs that require a custom header, and return JSON not HTML. You can use the new -H parameter to test it

```
sitespeed.io -H "{\"x-business-group-id\":\"http://aaa.bbbb.com/business-groups/b\",\"Accept\":\"application/json\"}" -r "$WORKSPACE/sitespeed-result/" -f "/home/nick/Documents/b-urls.txt"
 ```