added test for post calls to dpc
opened up /dpc url for post requests
re-purposed DPC model as a wrapper around a JSON file
implemented functional POST behavior for DPC API
renamed test_pgsql_installation file
applied small fixes to dpc model, updated its test
also renamed the other tests
Merge pull request #188 from bepetersn/v2.0-dev

VS 2.1.1 -- created one possible version of the DailyPopulationChanges model; integrated with GET and POST