Pipelet:
- Implement Plug class, base for Input and Output, methods:
  - _get_name()
  - error()
- Implement Input class, methods:
  - __fetch_source()
  - __fetch_source_all()
- Restrict Output class to no-query tree
- Implement Query_Tree_Output class with query tree methods
- Removed unused briged methods, and othe code cleanup
Merge branch 'master' of https://github.com/ConnectedSets/ConnectedSets