[SPARK-6696] [SQL] Adds HiveContext.refreshTable to PySpark

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Author: Cheng Lian <lian@databricks.com>

Closes apache#5349 from liancheng/py-refresh-table and squashes the following commits:

004bec0 [Cheng Lian] Adds HiveContext.refreshTable to PySpark

python/pyspark/sql/context.py

@@ -574,6 +574,15 @@ def _ssql_ctx(self):
     def _get_hive_ctx(self):
         return self._jvm.HiveContext(self._jsc.sc())
 
+    def refreshTable(self, tableName):
+        """Invalidate and refresh all the cached the metadata of the given
+        table. For performance reasons, Spark SQL or the external data source
+        library it uses might cache certain metadata about a table, such as the
+        location of blocks. When those change outside of Spark SQL, users should
+        call this function to invalidate the cache.
+        """
+        self._ssql_ctx.refreshTable(tableName)
+
 
 class UDFRegistration(object):
     """Wrapper for user-defined function registration."""