SCHEMA-295. Ability to override reader schemas in Kiji tables

- introduces CellDecoderProvider.
- refactor TestHBaseKijiRowData to get rid of deprecated constructor.
- HBaseKijiRowData has almost only one constructor.
- CellSpec now has a CellDecoderFactory.
- Test Avro records (Node/Edge) have proper default values.
- CellSpec supports "no reader schema" (ie. use writer schema).

issue: https://jira.kiji.org/browse/SCHEMA-295
review: https://review.kiji.org/r/289
SCHEMA-323. KijiReaderFactory to create custom readers.

- adds KijiTable.getReaderFactory().

issue: https://jira.kiji.org/browse/SCHEMA-323
review: https://review.kiji.org/r/284
SCHEMA-322. Restrict KijiRowData.getPager(family) to list the qualifiers in the family

- adds StripValueColumnFilter (cloned from StripValueRowFilter).
- update KijiPager documentation for map-type families.
- implement pager for map-type families, to retrieve qualifiers only.
- remove obsolete tests.

issue: https://jira.kiji.org/browse/SCHEMA-322
review: https://review.kiji.org/r/283
SCHEMA-339. Run unit-test against real HBase

issue: https://jira.kiji.org/browse/SCHEMA-339
review: https://review.kiji.org/r/315
Merge branch 'master' of https://github.com/kijiproject/kiji-schema