Add :cacheable option to reportable

This configures specific reports to always skip the cache. This is useful when the same report option is called with multiple conditions throughout the application. Without :cacheable => false, a report will return the wrong results when called with different conditions.

For example:

   Widget.hourly_report()           # Works as expected
   @box.widgets.hourly_report() # Uses the cache result from the *global* report above

This patch enables the above when :cacheable => false is specified:
    class Widget < ActiveRecord::Base
      reportable :hourly, :aggregation => :count, :grouping => :hour, :live_data => true, :cacheable => false
    end