Add support for labeling seed data.

This is a breaking change, as it changes the expected format of seed data files from a YAML Array to a YAML Hash, and requires that all seed data have an associated label.

The change includes
* Auto-generating IDs and UUIDs for each piece of seed data
* Allow for the linking of elements using lables instead of IDs
* Search for seed data using labels
Consistently using strings instead of symbols internally
Re-add ability to fetch by ID
Support user specified ids
Allow user to specify the type of ID to use for each seed data group
Updated CHANGELOG and README
Merge branch 'master' into seed_labels

Conflicts:
	CHANGELOG.md
Bump version for the eventual release
switch to the new rspec expectation syntax
Don’t process YAML data if it could not be loaded successfully
Allow user to specify the type of seed data in the value
Cap the length of a generated id
Added code to handle arrays of labels
Updated documentation