Add atomic add and remove role endpoints to HTTPClient.
Add atomic add and remove role endpoints to HTTPClient.

These are currently unused since their use is pretty limited.

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