Allows configuration of "lower number is better"

rebase of #67
Merge branch 'master' into lower-is-better-rebase

Conflicts:
	lib/tricle/metric.rb
	lib/tricle/presenters/metric.rb
change css classes to "good" and "bad"
Update "lower is better" readme section
re-remove metric
create percent_change_class method
correct+tweak lower->better instructions
Merge pull request #70 from afeld/lower-is-better-rebase

Allows configuration of "lower number is better"