create Report, which contains the metrics
support groups within Mailers
add another group to test mailer
fix mailer preview
Merge remote-tracking branch 'upstream/master' into group

Conflicts:
	config.ru
	lib/tricle/mailer.rb
	spec/spec_helper.rb
	spec/unit/mailer_spec.rb
require Group from Report
Merge remote-tracking branch 'upstream/master' into group

Conflicts:
	spec/unit/mailer_spec.rb
Merge pull request #36 from afeld/group

@fancyremarker => ability to specify groups of metrics