Added Extension Methods for WebSocket

Cached Channels keep a local cache of the members of that channel. This commit adds a synchronus method to access the cached user stores.

It also fixes a bug where the GetUser extension would return an IGroupUser by soft-casting to IGroupUser, which would always return null. ISocketUser does not share an inheritance with IGroupUser, so I now return IUser instead.

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

Added `Command.Synopsis`

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.

Remove all references to XKCD URLs in Invites

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.

Resolves #153

This is a breaking change!

With the introduction of Vanity URLs and the shortening of Invite URLs, the API appears to have removed the "xkcdpass" field of the Invite. 

Since this functionality was removed from the Discord API and thus obsolete, it has been removed from the Invite models.

Added Sharding support

Ignore CHANNEL_PINS_UPDATE