SPARK-1456 Remove view bounds on Ordered in favor of a context bound on Ordering.

This doesn't require creating new Ordering objects per row.  Additionally, [view bounds are going to be deprecated](https://issues.scala-lang.org/browse/SI-7629), so we should get rid of them while APIs are still flexible.

Author: Michael Armbrust <michael@databricks.com>

Closes #410 from marmbrus/viewBounds and squashes the following commits:

c574221 [Michael Armbrust] fix example.
812008e [Michael Armbrust] Update Java API.
1b9b85c [Michael Armbrust] Update scala doc.
35798a8 [Michael Armbrust] Remove view bounds on Ordered in favor of a context bound on Ordering.

(cherry picked from commit c399baa0fc40be7aa51835aaeadcd5d768dfdb95)
Signed-off-by: Reynold Xin <rxin@apache.org>