Adds a sync method to retrieve all cached IGuildChannels on an IGuild.

For users importing `Discord.WebSocket.Extensions`, there are now non-async extensions for GetUser.

These methods point to the appropriate method on the respective SocketgChannel.

Instead of implicitly soft-casting IGuild to SocketGuild, I added a method to soft-cast IGuild to SocketGuild, and throw an InvalidOp if it came up null for some reason.

GetUsers was changed from an IEnumerable to an IReadOnlyCollection to allow quick ".Count"-ing on the collection.

For users importing `Discord.WebSocket.Extensions`, there are now non-async extensions for Get(Type)Channel(s) and Get(Current)User(s).

These methods point to the appropriate method or member on SocketGuild.

Added Sharding support

Ignore CHANNEL_PINS_UPDATE

This gateway event is raised when a message in a GuildChannel is pinned or unpinned. Unfortunately, this event only contains a timestamp of when the pin was changed, making it effectively useless. As a result, it will be dropped.

Resolves #135

Ignore static constructors when creating types