Add headers arg to build_part

from Faraday 0.8 --> 0.9 upgrade
Add matcher descriptions to pacify rspec

if the tests fail without these matchers, rspec outputs very angry
warning messages about missing descriptions. It's annoying so shut it
up with these...
Test on proxy.uri instead of proxy

Proxies are no longer hashes but ProxyOptions objects, so just make
sure that the uri is right
Replace middleware calling with direct parsing

I don't feel like constructing a whole app+call+on_complete stub, so
forget it and directly parse the body
Update Faraday from 0.8 to 0.9
Initialize Middleware base class
Subclass standard mashify instead of our own

This lets us hook into the on_complete, parsing, etc.