kiji-schema: SCHEMA-800. Create method for building a map from a KijiResult.

issue: https://jira.kiji.org/browse/SCHEMA-800
review: https://review.kiji.org/r/1917
commit: https://github.com/kijiproject/kiji-schema/commit/1f13cd604573abaaf7f28fd4470f3d068b75b34e