Adds support for editing messages.

Wrapper now has an `.edit_message(message_id, text)` method.

MessageEvents now have an `.edit(text)` method.

It's possible that bad things might happen if you call this on a message
you can't edit; we don't have any handling for those errors and might
get stuck in an infinite loop failing to make an edit.

resolves #54
Merge branch 'editing' into test