* Fix null channel being passed in RequirePermission preconditions

* c#7 pattern matching

commit b7fb44a94f
Author: Hsu Still <341464@gmail.com>
Date:   Sat Sep 2 14:34:18 2017 +0800

    Fix more async naming violation

commit e6912e2d02
Author: Hsu Still <341464@gmail.com>
Date:   Sat Sep 2 14:23:04 2017 +0800

    Fix incorrect null xmldocs string

commit da8d23222d
Author: Hsu Still <341464@gmail.com>
Date:   Sat Sep 2 14:17:12 2017 +0800

    Fix CheckPreconditionsAsync

commit 992407407a
Author: Hsu Still <341464@gmail.com>
Date:   Sat Sep 2 14:07:12 2017 +0800

    Add Async suffix to abstract Task methods

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

This is a breaking change.

Adds a precondition that requires the bot has a specified permission (Resolves #211).

Renames RequirePermission to RequireUserPermission.