kiji-express: EXP-356. Create construct methods for ColumnInputSpec and ColumnOutputSpec.

These construct methods are to be used by kiji-express-cascading's builder classes:
ColumnInputSpecBuilders and ColumnOutputSpecBuilders.

Adds getTap as a static method on KijiSource companion object.

issue: https://jira.kiji.org/browse/EXP-356
review: https://review.kiji.org/r/1483
commit: https://github.com/kijiproject/kiji-express/commit/92db45e330480de758332a6afeeb1008ee76ffac