VS 2.1.1 -- DailyPopulationChanges (DPC) model, POST API, tests for both

## Summary of Changes

### API
- opened up '/daily_population_changes' URI for POST calls
- added test to make sure POST calls can be made, and that data POSTED can be retrieved

### Model
- re-purposed DPC model as a wrapper around a csv file
- made DPC model select a path for a database file behind the scenes, based on Flask's config and env variables
- added a few tests to make sure DPC model behaves as expected

## Usage

    >>> # Chooses a file behind the scenes; see 'ccj/config::get_dpc_path' for the implementation.
    >>> # Set the TESTING env. variable before any import statement, if you are testing.
    >>> from ccj.models.daily_population_changes import DailyPopulationChanges as DPC
    >>>
    >>> # The file is initialized here if it doesn't exist.
    >>> dpc = DPC()
    >>>
    >>> # You can check which file is being used by checking the private '_path' variable.
    >>> dpc._path
    '/tmp/dpc.json'
    >>>
    >>> # The file is empty currently.
    >>> dpc.query()
    []
    >>>
    >>> # The store() function expects a flat dict of key-value pairs, compatible with POST
    >>> entry = {'date': '2013-10-18', 'booked_male_as':'9'}
    >>> dpc.store(entry)
    >>>
    >>> # The query() function returns back a nested dict, to the API spec.
    >>> dpc.query()
    [{'Date': '2013-10-18', 'Booked': {'Male': {'As': '9'}}}]