Deprecates use/try methods

Part of: #18 (closed)

This deprecates the use and try methods and consolidates to control, candidate, and variant(:name) method names instead.

Use control instead of use, candidate instead of try, and variant(:name) instead of try(:name).

It deprecates using candidate(:name), and renames the old variant getter/setter to assigned, but continues to handle the old interface with a deprecation warning for the time being.