@afeld => Allow a group in a report to be specified as daily

Great gem @afeld !

What do you think about letting a group in a report be marked as 'daily', which would then include the following fields: Total Yesterday, Total and Pct Change from the Day Before That, and Total and Pct Change from 1 Week Ago.

I think the specification for daily needs to be done at the group level, as the headers for those metrics will be different.

So you can specify it like:

``` ruby
group "Daily Group 1", daily: true do
  metric TestDailyMetric
end

group 'Weekly Group 1' do
  metric TestMetric
end
```

Output looks like:

![screen shot 2014-04-25 at 6 08 41 pm](https://cloud.githubusercontent.com/assets/1457859/2806266/32dc3476-ccc6-11e3-9a3a-6508a7aaad2e.png)