Also changes target to NetStandard1.3

* Partial fix of #810, addresses critical connection issues.

* Implement fix for REST.

* Implement fix on RestChannel.

* Remove extra AddField overload in EmbedBuilder

* Remove AddInlineField()

* Add GetReactionUsersAsync(IEmote) overload

Resolves #730

* Remove obsolete GetReactionUsersAsync(string) overload

* Add NullableTypeReader.

Primitives now also load a NullableTypeReader<T> and any value types that get a typereader added will also have a NullableTypeReader<T> added for it.

* Remove unnecessary null check.

* Added docstrings.

* 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)

* Fix CreateGuildAsync not doing anything with the input stream for the guild icon.

Also fixes an issue with potential stream types that throw a NotSupportedException when checking its properties. [Apparently, they exist.](https://github.com/dotnet/corefx/blob/master/src/System.Net.Http.WinHttpHandler/src/System/Net/Http/WinHttpResponseStream.cs)

* Merged with old method

* Removed duplicate decl

* Throw when attempting to add or remove a member's EveryoneRole

This resolves #780

* Removed braces

* Update RestRole.cs

Fix everyone mention.

* Update SocketRole.cs

Fix everyone mention.

* I'm good at this, I swear.

Seeing as D.NET will warn you about an impending BadRequest if you try and send an empty field, why not make it warn about the impending BadRequest if you try and send a whitespace field?

This provides tests to ensure the following:
- Creating a Color
- Creating a Default Color
- Accessing a Color's Raw Value
- Accessing a Color's translated RGB values

This resolves #784

Style cleanup.

Style cleanups.

Remove extraneous overloads.

* Update client.cs

Let's not have the client be a local variable, hm?

* Update complete.cs

* Update complete.cs

* Update client.cs and complete.cs

Let's not have the client be a local variable, hm?

DiscordSocketClient and DiscordShardedClient's shared members now exist in this abstract class.

Fixed RestGuildUser not updating inherited RestUser properties

* Fix potential nullref in embedBuilder value setter

* Null check on footer iconUrl

* Adding checks for the other URL properties

* Adding IsNullOrUri extension

* Setting StringExtensions as internal

* Make ShardedCommandContext derive from SocketCommandContext

* Explicitly re-implement ICommandContext.Client