Make sure different recommenders have different namespaces by default.
Add Predictor.redis_prefix configuration option, which accepts a proc.
Merge branch 'key-namespace-flexibility' into prediction-boost

Conflicts:
	lib/predictor/base.rb
	spec/base_spec.rb
	spec/spec_helper.rb
Allow per-class redis key prefixing, and make global API consistent with it.
Make sure people can use redis_prefix([nil]) to get the old behavior.
One more spec to make sure arrays work as redis prefixes.
Update readme and changelog for redis_prefix changes.
Merge branch 'key-namespace-flexibility' into prediction-boost

Conflicts:
	lib/predictor/base.rb
Drop associative recommendations. Multiple revert of 507d4b2 and 3d2bfe0.

Conflicts:
	lib/predictor/base.rb
Have predictions_for support the :boost option even when there's no item_set.
Add the predictions_for :boost option to the readme.