Optimize mentoring-dataexport times by using values_list()

The bulk reading of the Answer objects to export have been converted to
a more efficient values_list() call
Merge pull request #1 from FiloSottile/optimize_dataexport

Optimize mentoring-dataexport times by using a raw SQL cursor
opaque-keys: Adds support for opaque keys

Inspired from https://github.com/edx/edx-ora2/pull/330
Merge pull request #2 from gsehub/opaque-keys

opaque-keys: Adds support for opaque keys
Specify contentType in ajax POST request.

Set content type to 'application/json' when POSTing JSON data to the server.

This fixes a bug where any occurence of the '??' string in POSTeda answer data
would get replaced with a random jsonp callback name generated by jQuery.

If content type is set to something other than the default www-form-urlencoded,
jQuery will not mess with the data.

cf: http://bugs.jquery.com/ticket/8417