SCHEMA-208, SCHEMA-211. KijiURI builder & exception.

SCHEMA-208. Convert KijiURIException to unchecked.
SCHEMA-211. KijiURIBuilder should default to "default" instance, not null.


tested with mvn verify.
I grepped the rest of the codebase; nothing explicitly 'throws KijiURIException' so this should not affect downstream project source code