[SQL] SPARK-1366 Consistent sql function across different types of SQLContexts

Now users who want to use HiveQL should explicitly say `hiveql` or `hql`.

Author: Michael Armbrust <michael@databricks.com>

Closes #319 from marmbrus/standardizeSqlHql and squashes the following commits:

de68d0e [Michael Armbrust] Fix sampling test.
fbe4a54 [Michael Armbrust] Make `sql` always use spark sql parser, users of hive context can now use hql or hiveql to run queries using HiveQL instead.