Allow projects to disable the sending of emails

Adds a global setting (SEND_PLANS_EMAILS) that allows projects to
disable all email sending from the package. Also includes some
documentation & a test for it.
Merge pull request #33 from klistwan/send_emails_setting

Allow projects to disable the sending of emails
Use django now() method to support application not working with timezones (prevent TypeError: can't subtract offset-naive and offset-aware datetimes)
Merge pull request #34 from andrzejpiasecki/timezone

Use django now() method to support application not working with timezones