Fetch unprocessed result files

Add a ``--raw`` flag to the ``fetch`` task to specify that
only unprocessed result files should be downloaded.

Add a ``raw_filename_url_pairs`` method to ``BaseDatasource`` as
a hook for providing only the unprocessed mappings between
standardized filename and URL.

Addresses https://github.com/openelections/core/issues/125
Merge branch 'dev' into 125-raw_fetch
s/raw/unprocessed/

Use "unprocessed" in method and variable names to avoid confusion
with the ``raw_url`` key in the datasource mapping dictionaries.

Addresses https://github.com/openelections/core/issues/125
Merge pull request #135 from ghing/125-raw_fetch

Merge datasource hook and task updates for fetching unprocessed files