Refactored CacheAncestry to support options, customized :relation_name

- Current spec status: 124 examples, 0 failures.
Fixed cache_ancestry :relation_name to acccept symbol values.

- Current spec status: 104 examples, 0 failures.
Fixed README
Minor documentation updates
Incremented version to 0.5.1 for release.

- Current spec status: 124 examples, 0 failures.