This commit gets rid of all the code that previously managed different
voice sessions in different guilds. This is because there is rarely ever
a need for this, and most bots that need this could do their own
keeping.
This change, although removes some features off of the package, adds a
lot of clarity on what to do exactly when it comes to connecting to a
voice channel.
In order to make the migration process a bit easier, an example has been
added which guides through using the voice.Session API.
