All drivers can switch to other drivers

[josephmancuso]

Closes #69

The idea here is that you should be able to quickly switch to other drivers from any driver. In other words you should be able to do something like:

def show(self, Mail):
    Mail.driver('mailgun').to(...)

even though Mail is an instance of a mail driver.

when previously you had to straight up use the MailManager class:

def show(self, Mail, MailManager):
    # send mail
    Mail.to(...)

    # switch drivers
    Mailmanager.driver('mailgun').to(...)

[coveralls]

Pull Request Test Coverage Report for Build 396

• 25 of 25 (100.0%) changed or added relevant lines in 11 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 85.776%

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Totals
Change from base Build 393:	0.2%
Covered Lines:	1188
Relevant Lines:	1385

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 429

• 25 of 25 (100.0%) changed or added relevant lines in 11 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 85.31%

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Totals
Change from base Build 424:	0.2%
Covered Lines:	1237
Relevant Lines:	1450

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💛 - Coveralls