Define SCPs supported contexts by host

[scaramallion]

At the moment when creating an SCP all requestors see the same presentation contexts. It would be nice if the supported contexts could be defined on a per-requestor basis (filtered by host/port/user/etc).

That way the services available to an SCU could be controlled by the SCP in a more user-friendly manner, allowing different levels of access.

It should be possible to use evt.EVT_CONN_OPEN with a handler to change the support contexts prior to association negotiation. Maybe add some optional plugin-like handlers?