Add spec test for bucket_logger with arbitrary name

This defines the creation and retrieval of bucket
contents using an arbitrary name, instead of a learner
id.
Migration: Add indexed string name for bucket loggers
Add routes and update controllers for bucket loggers w names

We can now post and get from
/dataservice/bucket_loggers/name/:name/bucket_contents
Add test for bucket_log_items for learner route

This tests the pre-existing functionality that we can add
multiple log items to a bucket logger specified by learner id
Add test for bucket_log_items for arbitrary name route
Add bucket_log_item routes and controllers for new named loggers

This adds the functionality of appending multiple log items to
a logger specified by an arbitrary name
Don't create new bucketLoggers on get requests; update test
Merge pull request #47 from concord-consortium/wip-bucket-logger-name

Create and retrieve bucket loggers using arbitrary names