Optimized getting all report element names.

The disclosure apps first asks for all names for a given fiscal year and period and a tag. This change projects the mongo query to the Name field. We're now at less than 700ms for the DOW30.

http://secxbrl-g-5.28.io/v1/_queries/public/api/report-elements.jq?fiscalPeriod=FY&fiscalYear=2013&map=Disclosures&onlyNames=true&tag=FORTUNE100&_method=POST