Add email support for newly approved vendors

Add a module for email functions, a template for the HTML email body,
settings to configure the outbound credentials.

Update save_existing() method to send an email when a vendor is newly
approved.

Add tests to make sure emails only get sent once.

Bring in mock library to create mock test objects.
Fix travis config

The newly added email validation in settings.py raises an
ImproperlyConfigured exception when the email hostname and password
variables haven't been set. We don't actually want travis to send
emails, so added some dummy data to avoid triggering this exception.
Merge pull request #260 from steventlamb/topic/add_email_support

Topic/add email support