* Add GetReactionUsersAsync(IEmote) overload

Resolves #730

* Remove obsolete GetReactionUsersAsync(string) overload

* Changed Guild#DefaultChannel to resolve the first accessible channel

Resolves #776

This change is inline with hammerandchisel/discord-api-docs#329

RestGuild#DefaultChannelId is now obsolete and will throw a
NotSupportedException.

* RestGuild#DefaultChannelId will fall back to the guild ID

Adding an exception here would be a breaking change, so this was agreed
to fall back to the previous behavior, which would just return the guild
ID.

IChannel.IsNsfw will now return false when being used on any channel
that is not an ITextChannel. When being used on an ITextChannel, this
will now account for the API flag, and fall back to the channel name.

(this is gross design, thanks discord)

* Update RestRole.cs

Fix everyone mention.

* Update SocketRole.cs

Fix everyone mention.

* I'm good at this, I swear.

* Add support for audit log reasons

* Made changes per discussion

* Add SocketRole.Members property

* Change Members to IEnumerable.

Remove IUser.CreateDMChannelAsync / Fix SocketGlobalUser.DMChannel

* Add IChannel.Nsfw, RequireNsfw precondition

* Refactored IChannel.Nsfw to IsNsfw, removed NsfwUtils

Per pull-request feedback

* proper nsfw channel check

* Removed GuildUserExtensions and moved the methods to IGuildUser and implementations

* Made changes per fox's suggestion: Change->Modify. New Modify overload.

* Oops

* Per Volt: reimplemented new endpoints

* Fixing broken docstrings

* I forgot that docstrings are XML

* Implemented atomic add/remove role endpoints

* Removed  so people aren't irked

* Added single-item role add/remove methods

Added support for reaction 'me' field

Automatic animated avatar detection