Add a REST interface

[mjordan]

As discussed in Islandora/documentation#1096.

[mjordan]

Basic REST interface is now included. See README.md for more info. I will continue to work on this feature under this issue.

[mjordan]

Note that currently, the creation of Bags using the REST interface is not truly asynchronous since the Bags are created during the HTTP response. To completely decouple the creation of the bags from the HTTP request, we probably need to accumulate node IDs in a queue and then run Islandora Bagger as a CLI in a cron job.

[mjordan]

5d010ee adds writing a queue file to the application's var directory that writes tab-delimited entries like this:

4       /home/mark/Documents/hacking/islandora_bagger/var/islandora_bagger.4.yaml

The first field is the node ID, and the second is the path to the YAML configuration file POSTed in the request.

[mjordan]

Closing since the basic REST interface for populating the queue works. Further additions to the REST interface should get their own tickets.