SCHEMA-169. Adds a KijiPager class and improves API.

This allows users to easily paginate and iterate over values from a column
while reading from a KijiTable. Users may not retrieve a KijiPager object
from a KijiRowData. The KijiPager is an Iterator of KijiRowData that can
then be used to access the values in the page.

Signed-off-by: Lee Sheng <lsheng@wibidata.com>