Merge pull request #369 from tiagoantao/master

Python 3: cmp to key
Cleaned up IterationAgent.
Changed AttributeDetail.editor_callable to AttributeDetail.editor.
Made AttributeDetail and AttributeManifest empty-initializable.
Implemented _attribute_manifest on Performer.
Updated TODO.
Updated Peformer._attribute_manifest.
Cleaned up Editor._get_attribute_editor().
Cleaned up Editor._get_attribute_editor().
Merged master.
Implement _item_creator_class on InstrumentInventory.
Wrote InstrumentInventory editor tests.
Merged master.
Wrote editor tests for Performer.
Cleaned up InstrumentationSpecifier initializer implementation.
Implemented _attribute_manifest on InstrumentationSpecifier.
Wrote editor tests for InstrumentationSpecifier.
Ported instrumentation tests.
Removed PerformerEditor.

Use Editor instead.
Removed InstrumentationEditor.

Use Editor instead.

MILESTONE: only two editor classes remain:

    * Editor
        * ListEditor

These two classes will be retained.