Switch to 'modular' sinatra app
Inject PostMailConfig.
Build request body for maildrill.
mailgun_mailer_spec looks like mandrill_mailer_spec
Validate presence of keys.
validator returns false if any key is blank
Validate email addresses.
Fix halt test. Remove router.
Move mandrill url from ENV to ManDrillMailer
Fix mandrill integration test and mailer.
Build proper mailgun url.
Move all access to ENV to PostMailConfig
Change PostMailConfig to static class
Add configuration validation.
Add lost to README.md