Adapted PES/OJA code from cleanup_learning branch
Fooling around in the encoder space
Cleaned up LearningRule classes and added docstrings
Completed learn_square.ipynb using PES
Added unit test for PES rule
Removed OJA's rule
Removed some unnecessary build state used by oja's rule
Small fixes in naming and comments
Signal label ONE -> PES:one
Merge branch 'learning_pes' of https://github.com/ctn-waterloo/nengo into learning_pes

Conflicts:
	nengo/builder.py
	nengo/objects.py