added 2004 district 20 correction
Remove duplicate call to handle_task

handle_task() was being called twice in datasource.mappings task.

Spotted by @tothebeat and someone else at the hackathon.  Thanks!
Merge remote-tracking branch 'openelex/dev' into dev
Use bulk inserts and cached lookup for created models

To speed things up, use bulk inserts to create new
Contest, Candidate and Result records. Generate keys and use
a set to avoid duplicates.

In order ot create lookup keys, I had to improve
RawResult.candidate_slug and RawResult.contest_slug. I also had
to work around a case where some results had a (seemingly)
erroneous "Office District" field for offices that shouldn't have
districts, like President. See #80.

Finally, add logic to set the ocd_id for the result records. In
the case of precincts, I decided to pass the parent county of the
district (which is available at the load stage in the mapping)
to the transform stage via an expando field, RawResult.count_ocd_id.
It seemed wasteful to have to look this up since the precinct to
county relationship isn't otherwise available at the transform
stage.

Addresses #49