Implement multitenancy

Depends on #6.

This implements multitenancy using the [`acts_as_tenant`](https://github.com/ErwinM/acts_as_tenant) gem.

Might not want to merge until @wangqiang1208 finishes his alternative implementation, for comparison purposes.
Use 2 spaces for indentation in a hash, relative to the start of the line where the left curly brace is.