SCHEMA-871. Implement get() and getScanner() for AsyncKijiTableReader.

Use the new KijiResult classes to implement the get() and getScanner() methods
in AsyncKijiTableReader.

issue: https://jira.kiji.org/browse/SCHEMA-871

Conflicts:
	kiji-schema-async/src/main/java/org/kiji/schema/impl/async/AsyncKijiReaderFactory.java