include email as user property

[acaire]

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've been mindful about doing atomic commits, adding documentation to my changes, not refactoring too much.
• I've a descriptive title and added any useful information for the reviewer. Where appropriate, I've attached a screenshot and/or screencast (gif preferrably).
• I've written tests to cover the new code and functionality included in this PR.
• I've read, agree to, and signed the Contributor License Agreement (CLA).

PR Summary

Our use case entails having Slack as a source of truth for users, so we'd like to keep track of user e-mail addresses. This PR adds an email property to the user object.

[coveralls]

Coverage increased (+0.06%) to 61.328% when pulling 55da0ea on acaire:master into c36dbf4 on slackhq:master.

[ghTravis]

I'm also looking for this functionality!
+1

[acaire]

Closing in favour of #173 due to deleted fork