Support for "conversation" responders.

Conversation responders check three things on every message they process:

1. Where the message was seen (public, or private)
2. Who spoke
3. Who they addressed, and whether that was the bot itself
4. What they said, split into words for parsing convenience

Responders get a convenience callback that correctly directs replies based on
those facts.
Added a `version` conversational command.

This command interrogates "environments" over HTTP, and reports back on version
numbers found in their headers. Environments must be configured in
`alicorn.conf` and should return at least one of `X-Version` and `X-Git-Version`
headers.

Alicorn now requires `requests`.