Added configuration options to Mongoid::SleepingKingStudios::Tree

- incremented version to 0.3.0
- current spec status: 62 examples, 0 failures
Added meta-customisation for Tree :parent and :children relations

- Incremented version to 0.3.1
- Current spec status: 72 examples, 0 failures.
Incremented version to 0.3.1