Properly require "coveralls" before all library files
Separate callback invalidations from invalidate_garner_caches
Rewrite strategies as classes with instances
Implement SafeCacheKey, with sub-second resolution, as the newest, safest cache key
Refactor strategies to be inheritable singleton classes
Standardize `self.method` instead of `class << self` except where necessary
for class method aliasing.
Remove unnecessary subject calls from spec
Extract safe_cache_key method out and put its logic into the strategy
Add caching for Mongoid::Document at the class level

This implements `cache_key`, `updated_at` and `touch` at the
class level simply by delegating to the most recently updated
document in the collection.
Add specs for the corner case cited by @dblock on #28
Update CHANGELOG and README
Properly exclude spec files from coverage
Fixed link to CONTRIBUTORS.
Fixed link to LICENSE.
Fixed copyright and year.