fix: make Message#author lazy

[biqqles]

Summary

Fixes #117. Calling Message#author will still result in 'Unknown Member' being printed if the member does not exist (don't think there's a way to hide this? It is kinda annoying), but it won't hang instantiators of Message such as Channel#history.

This is a bit rough right now but functional, will make a cleanup pass when I have more time.

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Fixed

• Reimplemented Message#author as a method instead of an attribute

[swarley]

Thanks for the PR! Just some minor things to work through before we get it merged