[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.
We need to output something here.  IIRC, Hive does not actually output the results of a `SET` as a relation, but instead prints it directly to the screen.  So, we can decide the semantics for the output schema.

I think the question is do we want to output the results as a single column, or separate the key and value?  I tend towards the latter.