[#37] Switch default adapter: net-http-persistent

Sure, this ropes in another gem dependency, but it's a good one.  Also in my
very unscientific tests, it speeds up repeat access to the API by about 4x.
readme update on http default
[#65] failing test to reproduce problem
[#65] Handle non-synchronous responses correctly
[#65] Parse JSON bodies of parallel responses

Can't use the Faraday-Middleware gem for this anymore, and it's meta-programmed
up enough that fixing it to support non-synchronous requests would be painful.
So, we do our own JSON rendering.
documentation fix
Merge branch 'mattly/37-faraday-adapter'
Merge branch 'mattly/65-parallelism'

Conflicts:
	orchestrate.gemspec