Implement an email function for users.

[pudo]

We need an email function in CKAN to accept messages sent to users. The basic signature will be:

• mail_user(user_obj, subject, body, mime_type='text/plain', headers={})

This has a number of use cases:

• Retrieval of lost passwords
• E-Mail confirmation

Finally, the mail function should be exposed in the API for sysadmin clients. This way we can have scripts traverse CKAN for 404s, invalid data or missing fields and ping users about that automatically (requires traversal by revision, not package, to get the associated users).