Simplified usage with inheritance + possibility of registering multiple handlers in one call

Hi,

I've made some changes that improves usage of `Observable`. 

New features:
  - possiblity of registering multiple handlers in one call of `once` and `on` methods
  - same for unregistering

Minor enhancement:
  - you can easily extend `Observable` class without implicit call of `Observable.__init__`
  
Also I've moved exception classes outside `Observable` for easy importing.

Could you check these improvements and release new version, please?