implement multiple instances

Hi,

in my environment, there is a demand for multiple instances of mrbs, being one booking system for rooms, one for media, one for tests.

In each one different people need to be admins, editors, viewers.

I tried to implement it and created a pull request on github.

The link is: https://github.com/arborrow/moodle-block_mrbs/pull/7

I would be happy if you could take a look on it. I will be happy to make adjustments.

Frank Schütte