[SPARK-1508][SQL] Add SQLConf to SQLContext.

This is still work in progress. (Potentially incomplete) List of things to do:

* Support set commands for `hql()`.
* Hook this into HiveContext as well.
* Run an end-to-end test on a cluster.
Also need to fix [here](https://github.com/concretevitamin/spark/blob/sqlconf/sql/core/src/main/scala/org/apache/spark/sql/execution/Exchange.scala?pr=%2Fapache%2Fspark%2Fpull%2F956#L91).