Fixes predicate method specs
Initial pass on adding century and decade precisions
#decade and #century now return integers
Updated tests to test for booleans, not falsey/truthy
Added tests for checks with explicitly set parameters, removed explicit #century? and #decade?
Merge branch 'CenturyPrecision' of https://github.com/workergnome/date_time_precision into CenturyPrecision

Conflicts:
	lib/date_time_precision/lib.rb
	spec/date_time_precision_spec.rb