Make tree classes register themselves.
Adapt testing classes to tree autoregistration.
Merge pull request #29 from GaretJax/autoregistration

Autoregistration
Merge branch 'develop' into left-right

* develop:
  Adapt testing classes to tree autoregistration.
  Make tree classes register themselves.
Use sqlalchemy's propagation support instead of registering every single
subclass.
Remove non-working implementation
Move TreeManager to events and implement parent expiration.
Make a default tree manager importable from the package.
Update tests for the new implementation.
Follow correct naming convention.
Add some documentation.