Introduce LearnerND, BalancingLearner.from_product, and several bug fixes.

• Introduce LearnerND (beta)
• Add BalancingLearner.from_product (see learner.ipynb or doc-string for useage example)
• runner.live_info() now shows the learner's efficiency
• runner.task.print_stack() now displays the full traceback
• Introduced learner.tell_many instead of having learner.tell figure out whether multiple points are added (#59)
• Fixed a bug that occured when a Learner1D had extremely narrow features

And more bugs, see v0.4.1...v0.5.0