added /data_sources endpoint
changed endpoint to /agg
add notice_types to /agg
Merge branch 'master' into data_source_endpoint
Break aggs out into separate endpoints, and add one with data_source -> notice_type nested.
Merge pull request #151 from 18F/data_source_endpoint

Issue #57: Add /agg endpoints to show unique data_sources, notice_types, and notice_types by data_source.