Feature new field

To avoid a nasty data manipulation at the web side, we are adding here a new field, *date_group*, that does the dirty work.
Then we can do something like:
```
db.hdapi_raw.aggregate( 
	{'$match':{'time_bucket': '2014-12-05-day'}}, 
	{
		'$project':{ 
			response_time':1, 
			date_group':1} 
	}, 
	{ 
		$group': { _id':'$date_group', avg':{'$avg':'$response_time'} } 
	} 
)
```
Please @haplo, @gasull, @rmariad, @thomasrockhu take a look at this.
It's a backward compatibility matter but thinking about it, we're not using **-minute** item nowhere.