Add Client

Currently, the Client can only be configured, it doesn't do anything
else.
Lower maximum ABC complexity
Add RequestSigner class
Add Faraday RubyGem as a dependency

Downgrade Faraday to 0.8.9 to be compatible with VCR
Add VCR RubyGem as a development dependency
Add Pete::Client#connection
Add Pete::Interface::HealthCheck
Parse JSON in responses using Faraday middleware