FR: Gmail Integration

As somebody who wants to use web-based gmail and keybase, a real solid keybase/gmail integration would set this system apart from all the others. Signing/encrypting + contact integration and the ability to be notified when my contacts join Keybase (kind of like the way google+ works now).

-ajp

The issue describes a broad Gmail and Keybase integration involving signing or encrypting mail, contact integration, and notifications when contacts join Keybase. No files, tests, or entry points are named, so first clarify the scope and locate the existing Gmail, messaging, and contact-integration code before defining a testable implementation boundary.

Written by the indexing model from the issue text.