Use condensed FactoryGirl syntax

FactoryGirl.create -> create
FactoryGirl.build 	 -> build
Add Conversation#mailbox/.match_mailbox

Both Account and Conversation now have #mailbox methods:
@account.mailbox #=> 'slug@helpful.io' (Mail::Address object)
@conversation.mailbox #=> 'slug+number@helpful.io' (Mail::Address object)
And .match_mailbox and .match_mailbox! (raise exception if not found) methods which try to match a string back to the Account/Conversation.
Update Concierge to support Conversation#mailbox addresses