Add mocha for nicer mocks/stubs
Send card and comment emails to involved users only
Add subscriptions to Board model

* Update Comment accordingly
* Update Card accordingly
Let the users choose their preference
Move recipients retrival out of Mailer
Use mocha consistently
Avoid assertions that appear on other tests
Extract common behavior for system notifications
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Merge pull request #30 from catchup/fine-grained-mailing

Fine grained mailing