Add queues to mail

[josephmancuso]

This PR adds the ability to use a new queue() method on mail sending.

previously we could do this:

def show(self, mail: Mail):
    mail.to('email@email.com').send('message')

Now we can optionally pass the queue method:

def show(self, mail: Mail):
    mail.to('email@email.com').queue().send('message')

[coveralls]

Coverage decreased (-0.4%) to 89.448% when pulling e2a100a on add-queues-to-mail into b04b01d on develop.