kiji-schema: SCHEMA-810. Hide KijiColumnName constructors behind a factory method

issue: https://jira.kiji.org/browse/SCHEMA-810
review: https://review.kiji.org/r/1909
commit: https://github.com/kijiproject/kiji-schema/commit/3e60e109b4659cfd9bf16e0c4c4c5a1f61433aa9