Carefully document how to send email without gmail

[cespare]

• Ensure that sending email using Pony but not gmail works without code changes (only configuration)
• Make sure this is well-documented
• Be sure to point out that sending email not through gmail will look like spam and users will have to whitelist it

[cespare]

This fixes #314.

[philc]

Pull request #379 is/was related to this.

The first question to be answered is:

1. Should we expose Pony configuration options directly, or have our own email configuration interface over Pony's interface?
2. What does that look like (assuming we continue configuring barkeep via environment.rb) for the gmail and non-gmail case?