Messaging: Add send_at param, update src to psr-4, clean up message test a bit

Reviewers:
@be-dmitry 
@mikeklein13 
I feel like if you're using a library without reading docs on either the API or the client library which both explicitly state this format (which i suspect really just means "parseable by `strtotime`" on Mandrill's end) you probably have bigger problems on your hands :)