ACCUMULO-1661 Handle empty column family correctly for AccumuloInputFormat

Signed-off-by: Eric Newton <eric.newton@gmail.com>
ACCUMULO-2227 / ACCUMULO-2228 Update randomwalk README with HA warning

Hadoop 2.1.0 includes better retry / failover handling than prior versions. This
commit adds a warning to the randomwalk README advising testers to expect more
failures exercising HA under Hadoop versions before 2.1.0.
Merge branch '1.4.5-SNAPSHOT' into 1.5.1-SNAPSHOT