Document data format for 2002 MD data

Make fields list passed to csv.DictReader align with the field
names on RawResult for easier construction later.

Also remove unused imports.

Addresses #49
Move setting of primary party out of _build_meta_fields()

The way that the row is fetched will vary from file to file, making
it hard to move this method to BaseLoader if we want to do that
in the end.

Addresses #49
s/_build_meta_fields/_build_common_election_kwargs/

Rename MDBaseLoader._build_meta_fields() to
MDBaseLoader._build_common_election_kwargs().

This is because we're ultimately building keyword arguments that
will being passed to the RawResult constructor.  Also, the _kwargs
naming convention is used in other similar cases.

I chose not to use the term "meta" because the keyword arguments
we're building include some information that will be in contest
records.  They're different than the fields labeled as meta in
models.  Ultimately, what they share is that they're common to a
particular election.

Addresses #49