Create a attendance tapping agent as a user

[mcflyhalf]

This user/agent represents a card reader or camera in a classroom. It should be able to mark student attendance but with stringent rules e.g:

• It can only mark attendance in a particular room (The room where it is located)
• It can only alter attendance for a class that is ongoing (cant alter attendance in the past or future)

These would require #27 to already be implemented so that the framework for implementing rules already exists.

[mcflyhalf]

To be done in far future when app includes physical device presence in a classroom

[mcflyhalf]

Will require creation of a new type of privilege in the User model