Migrate to the pelias-document module.

package.json, lib/dataset_import.js
	-Add the `pelias-document` module as a dependency, and rewrite
	`peliasSuggesterMapper()`, used by `createImportPipeline()`, to
	create `Document` objects instead of raw objects. Improves
	maintainability/compatibility in the future, should the
	underlying object schema ever change.