Add setUser

[dcramer]

// Set the user data
Raven.setUser({
  email: 'dcramer@example.com'
});

// Clear the user data
Raven.setUser();

This would fill in sentry.interfaces.User

[mattrobenolt]

What's the interface look like for this?

[dcramer]

It's arbitrary information, but we should document examples sending an email address:

http://sentry.readthedocs.org/en/latest/developer/interfaces/index.html#sentry.interfaces.User

[mattrobenolt]

Added with cc0a471. GitHub didn't like my rebased commit message.