Bot should interact with key servers

Right now it requires users to attach their public key. Based on Seth's comment in #3 I think it should continue to require this, but it should also search key servers for the sender's email address and report back what it found.

The issue names no files or tests; start by reading the referenced discussion in issue #3 and locating the code that handles attached public keys and sender email addresses. Done means the bot still requires an attached key, searches key servers for the sender's email address, and reports what it finds.

Written by the indexing model from the issue text.