Define AnswerDistributionWorkflow.

Combines AnswerDistributionOneFilePerCourseTask and
AnswerDistributionToMySQLTaskWorkflow into a single workflow.
Allow selecting courses on event exports.
Merge pull request #44 from edx/brian/answer-dist-all

Define AnswerDistributionWorkflow.
Refactor daily course enrollment
Merge pull request #25 from edx/gabe/refactor-enrollments

Refactor daily course enrollment
fixup! Allow selecting courses on event exports.
Merge pull request #42 from edx/select-exported-courses

Allow selecting courses on event exports.