Add permission checking methods

[maxwai]

Add methods to check if a user:

• has a specific role
• has one of a list of roles
• has none of a list of roles
• is an admin (check if any of his roles have admin permissions)
• is the server owner

Add method to check if a message was send in the main guild.

Add method to check if a message was send in one of a list of channels

Add method to check if guild, where message is send, has completed setup (check if guild is in database)