Add PermissionOverwrite.is_empty to query empty state of an overwrite.
Add PermissionOverwrite.is_empty to query empty state of an overwrite.

Fixes #382

mgardne8/discord.py,Harmon758/discord.py,Harmon758/discord.py,khazhyk/discord.py,Rapptz/discord.py,rapptz/discord.py,imayhaveborkedit/discord.py,variablehair/Eggplantato