Add class for managing single or multiple named API keys.
Only send messages if the host matches.

If a single key is configured, then the host is deemed to always match
and a message is always emitted.
Update README with description of how to use named API keys.
Switch from Guava Optional to JDK Optional.
Use JDK Optional features.
Merge pull request #4 from gkopff/feature/named-keys

Feature/named keys