Add initial benchmarking framework
Eliminate separate configuration parameters for Mongoid-specific binding strategies
No longer require all support libraries up front in spec_helper.rb
Improve performance of class and virtual bindings with SafeCacheKey strategy

* Re-use the _latest_by_updated_at object for class bindings, to eliminate
  multiple queries for the same object.
* Memoize the partial database object fetched for virtual bindings.
Small changes to benchmark suite
Improve usefulness/accuracy of StrategyBenchmark

* Add R parameter, for number of recursive database queries per cache block.
* Properly warm cache before all warm cache calls.
Add { safe: true } back into Mongoid test mode options
Updated README and CHANGELOG
Merge pull request #34 from fancyremarker/benchmarking

Benchmarking for binding strategies