kiji-schema: SCHEMA-882. Allow Easy Access to Materialized KijiResult

Added static helper function for getting MaterializedResult

It is helpful to be able to get a MaterializedResult contents easily
since this is not implementation dependent.

issue: https://jira.kiji.org/browse/SCHEMA-882
review: https://review.kiji.org/r/2074
commit: https://github.com/kijiproject/kiji-schema/commit/56979fcf36b7b9748b9fa8c68e492c286484d1c1