SCHEMA-187. CellSpec must handle generic records (eg. when generated Avro classes are not available)

This change fixes the reader side of SCHEMA-187 only.
The writer side will be fixed in another issue/change.

- CellSpec now has a "no reader schema" mode (ie. use writer schema).
- Cell decoders use the writer schema/generic records when specific classes are not available.
- No unit-test added yet (we cannot write generic records yet).
I tested this change manually using the KijiMusic end-to-end test, running kiji scan on the
songs table without the SongMetadata specific record on the classpath.

issue: https://jira.kiji.org/browse/SCHEMA-187
review: https://review.kiji.org/r/290