Added a new LearningRule object, which computes parameter updates from
gradients. Implemented `Momentum` and `AdaDelta` learning rule.
Added more documentation to LearningRule objects.
Introduced bugs when preparing for pull requests.
Added unit-test for new learning rules.
Moved MomentumAdjustor from sgd.py to learning_rule.py.

MomentumAdjustor will work with both old and new interface of momentum, until
init_momentum is deprecated.
Updated YAML files to use new learning_rule.Momentum interface.
Small bug fix to make MomemtumAdjustor work with bold old (init_momentum=) and
new (LearninRule) interfaces to momentum.
Misc. fixes following comments on PR introducing the learning rule mechanism.
https://github.com/lisa-lab/pylearn2/pull/420
Merge pull request #420 from gdesjardins/adadelta

Momentum and AdaDelta as LearningRule