Add non-aggregated export.

### Overview

The purpose of this pull request is to allow the ability to export actual event values similar to how a SQL query returns individual rows. The output of non-aggregated selection fields always goes to a single object within the `_` field. Non-aggregated selection still supports grouping and namespacing. This also works with all data types (string, factor, integer, float, & boolean).

### Example

```skyql
SELECT name, timestamp, action

# =>
{
  "_":[
    {"name":"Bob", "timestamp":1325376000, "action":"purchase"},
    {"name":"Bob", "timestamp":1325376060, "action":"chargeback"},
    {"name":"Susy", "timestamp":1325376363, "action":"view home page"},
  ]
}
```

/cc: @TheHydroImpulse @boourns @orenmazor @snormore