[SPARK-5872] [SQL] create a sqlCtx in pyspark shell

The sqlCtx will be HiveContext if hive is built in assembly jar, or SQLContext if not.

It also skip the Hive tests in pyspark.sql.tests if no hive is available.

Author: Davies Liu <davies@databricks.com>

Closes #4659 from davies/sqlctx and squashes the following commits:

0e6629a [Davies Liu] sqlCtx in pyspark