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
fixed date formatting for wv url_paths
fetch pre-2008 WV files from github repo
WV csvs do not have winner flag
added pre_processed_url to datasource attrs
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
Allow fetching of both processed and unprocessed results

Update filename_url_pairs() to use the link to the preprocessed
CSV results from GitHub, if available.

Return the unprocessed PDF URLs via unprocessed_filename_url_pairs().

Addresses https://github.com/openelections/core/issues/125
removed data_url from url_paths for wv
better check for wv special boolean
Merge branch 'dev' into 124-ar_datasource