Set a default and customizable User-Agent

In an attempt to be good citizens and provide context when debugging, this overrides the default user-agent  set by em-campfire. 

To actually do anything this requires the merging of https://github.com/wjessop/em-campfire/pull/1 and bumping the required em-campfire version to include it.

Related note, seeing a lot of test failures from 763b7a50c70c639993957f0ca431610396f421ad.