Add atomic keyword argument for member role editing operations.
Add atomic keyword argument for member role editing operations.

This affects:

* Member.add_roles
* Member.remove_roles

This is the main attempt of fixing long standing bugs like #56. Since
cache consistency is too hard to ask for due to eventual consistency
and this generally being one of the only main roadblocks that the
cache ruins, it's best to just implement it in terms of the atomic
endpoint instead.

Fixes #56

Rapptz/discord.py,khazhyk/discord.py,Harmon758/discord.py,imayhaveborkedit/discord.py,Harmon758/discord.py,rapptz/discord.py