Add JavaDoc to the configuration settings; deprecate the RuleChainWalker

This release is focused on resolving a couple of minor issues, both related to a lack of documentation.

• I added JavaDoc to the constants of the JUnitConfig class to resolve #85.
• I deprecated the RuleChainWalker class to resolve #84.

The latter change was informed by the fact that the @rule annotation now includes an order parameter that provides the same sort of invocation order control that the RuleChain class gives.