Add validators for MD 2006 general election

Addresses https://github.com/openelections/core/issues/36
Enable history, autocomplete in shell task

I totally stole this from Django's
``django.core.management.commands.shell``.  I added a comment and
the Django license to specify this, but we might need to address
this in a different way if the licenses are incompatible.
Validators for 2008 Maryland elections

Contest, candidate and result count valdiators for the Maryland's
2008 primary and general elections.

Also add a transform to compress multiple rows of
"Uncommitted to Any Presidential Candidate" results in the
Democratic primary into a single result per legislative district.

Addresses https://github.com/openelections/core/issues/36
Validators for Maryland 2010 elections

Contest, candidate count and result count validators for
Maryland 2010 primary and general elections.

Addresses https://github.com/openelections/core/issues/36
Merge branch 'dev' into 36-md_validators
Add Maryland 2012 election validators

The 2010 redistricting process changed the county to congressional
district mappings, so I had to define the new mapping in
``us.md.jurisdiction``.  In addition, I tried to standardize the
naming conventions of the mappings variables in the jurisdiction
module.  I also made the various jurisdiction definitions class
attributes of the election classes to make access within methods
the same.  If a mapping varies between elections, it can be specified
in the class definition.

Addresses https://github.com/openelections/core/issues/36
Handle case when there are zero candidates

Occasionally, a party has no candidate in a closed primary.
Try to handle this case more gracefully.

Addresses https://github.com/openelections/core/issues/36
Merge branch 'dev' into 36-md_validators