Ask email from user if it's not provided by auth provider

[roll]

Overview

For now we're going to accept registrations without emails (#84) because for example github user profile could have only private email addressed. But if it will be needed by future features like notifications we should to collect this information from user directly.

[pwalsh]

I don't see why we need it? As a general idea I get it, but I don't see a well formed use case to email the user, def. not in the near future.