Fix attempting to inject into static properties

It didn't make much sense

* Add SocketRole.Members property

* Change Members to IEnumerable.

* Add various property validation in EmbedBuilder

* Embed URI changes

Changes property types for any URLs in Embeds to System.URI.
Adding field name/value null/empty checks.

* including property names in argumentexceptions

* Adds overall embed length check

This resolves #702

* Add IUser#SendMessageAsync extension

* Add ConfigureAwait

* Add grouping of preconditions to allow for flexible precondition logic.

* Fix checking Module Preconditions twice (and none of the command's own)

* Fix command preconditions group 0 looping over every other precondition anyway #whoopsies

* Use custom message when a non-zero Precondition Group fails.

* Fix doc comment rendering.

* Refactor loops into local function

* Considering a new result type

* Switch to IReadOnlyCollection<T> and fix compiler errors

* Revert PreconditionResult -> IResult in return types - Change PreconditionResult to a class that PreconditionGroupResult inherits.

* Feedback on property name.

* Change grouping type int -> string

* Explicitly use an ordinal StringComparer

* Full stops on error messages

* Remove some sillyness.

* Remove unneeded using.

Change wording of permission preconditions

Forgot a var >_>

Gladly taking suggestions for a better comments.

Also fix an issue where RequireBotPermission may throw if used in a
non-guild channel which required guild permissions.

Remove IUser.CreateDMChannelAsync / Fix SocketGlobalUser.DMChannel

Remove wrong parameter from FFMPEG audio example

Add newest verification level

Fixed exemple calling old non existing function.

Update the example precondition to use IServiceProvider

Update cmd docs to use IServiceProvider

* Update intro.md

* Update intro.md

* Update intro.md

* Update intro.md

* Update intro.md

* Update intro.md

* Update intro.md

Added/clarified some comments in structure.cs

Fix spelling

Users must have a verified phone on their Discord account.

http://i.imgur.com/BexDgzS.png

Update the quickstart structure example to rc3.