[SQL] Make it possible to create Java/Python SQLContexts from an existing Scala SQLContext.

Author: Michael Armbrust <michael@databricks.com>

Closes #761 from marmbrus/existingContext and squashes the following commits:

4651051 [Michael Armbrust] Make it possible to create Java/Python SQLContexts from an existing Scala SQLContext.

(cherry picked from commit 44233865cf8020741d862d33cc660c88e9315dea)
Signed-off-by: Reynold Xin <rxin@apache.org>