spec connection/ping to sandbox service

* Added a #ping method to test connectivity to the CardConnect REST API
* Changed Client#initialize to accept a hash for config
Here's some code for this from VirtualBank, however I've meant to refactor it to not be so ugly.

# Create a .test.env and a .development.env for your different local environments
require 'dotenv'
paths = %W(.env .#{ENV['RAILS_ENV']}.env).map { |name| Dir.pwd + '/' + name }
Dotenv.load(
  *paths
).each { |k, v| ENV[k] = v }