Add tracing support for faraday requests

[sl0thentr0py]

Right now we just have Net:HTTP support for outgoing http requests and if Faraday uses some other adapter we don't get any spans. https://github.com/lostisland/awesome-faraday/#adapters

[nhorton]

This would be really nice to get sometime soon. Even if it is not fully tested and such, having an example that generally works would go a long ways.