refactor aggregation methods into mixin

allows them to be overrriden easier
add note about to readme about aggregation methods
require ActiveSupport::Concern from Aggregation mixin
Merge pull request #69 from afeld/aggregate

refactor Metric presenter into a mixin on the Metric
Merge branch 'master' into lower-is-better-rebase

Conflicts:
	lib/tricle/metric.rb
	lib/tricle/presenters/metric.rb