Update RSpec dependency to ~> 2.13.
Drop state before each RSpec example.
Add Postmark.deliver_messages method, rename Postmark.send_through_postmark method to Postmark.deliver_message, alias it as Postmark.send_through_postmark for backward compatibility.
Add unit tests for Postmark module.
Fix an extra retry attempt caused by invalid condition in ApiClient#with_retries method.
Move Postmark::TimeoutError instantiation to Postmark::HttpClient.
Add specs for Postmark::ApiClient and Postmark::HttpClient.