KIJIMR-19. Improve testing framework.

 - KijiClientTest provides a default test configuration.
 - KijiClientTest provides a local temporary directory.
 - KijiClientTest exposes a test ID with class and method name.
 - Force a GC after each test, to trigger resource finalization.

Signed-off-by: Lee Sheng <lsheng@wibidata.com>
SCHEMA-118. Cleans up KijiTableLayout API.
Tiny javadoc change (thanks Kimball)