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