To many files.

* Fix comments to comply with third-person commenting style
+ Various attributes
+ Various Command-related objects
+ Many more

* Apply consistency to attributes in the commands extension

This resolves #527.

Not sure if I missed any, putting this up for review.

* Allow preconditions to be used multiple times (for use with groups)

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

The new RequireContextAttribute works just like RequireDM/RequireGuild,
but is more powerful as developers can specify multiple 'contexts' for
their command to require using the ContextType flags.