Introduce two additional RSpec execution filters.
Add #senders, #servers, #get_senders_count and #get_servers_count to Postmark::AccountApiClient class. Introduce improved API to work with collection-type resources. #41
Add #messages, #get_messages_count methods to ApiClient, refactor related #get_messages and #extract_message_path_and_params to support the new underlying API. #41
Add separate unit tests for Postmark::Client#find_each, this allows us to simplify tests for Postmark::AccountApiClient. #41
Fix unit tests on Ruby 1.8.7.