[SPARK-7204] [SQL] Fix callSite for Dataframe and SQL operations

This patch adds SQL to the set of excluded libraries when
generating a callSite. This makes the callSite mechanism work
properly for the data frame API. I also added a small improvement for
JDBC queries where we just use the string "Spark JDBC Server Query"
instead of trying to give a callsite that doesn't make any sense
to the user.

Before (DF):
![screen shot 2015-04-28 at 1 29 26 pm](https://cloud.githubusercontent.com/assets/320616/7380170/ef63bfb0-edae-11e4-989c-f88a5ba6bbee.png)

After (DF):
![screen shot 2015-04-28 at 1 34 58 pm](https://cloud.githubusercontent.com/assets/320616/7380181/fa7f6d90-edae-11e4-9559-26f163ed63b8.png)

After (JDBC):
![screen shot 2015-04-28 at 2 00 10 pm](https://cloud.githubusercontent.com/assets/320616/7380185/02f5b2a4-edaf-11e4-8e5b-99bdc3df66dd.png)

Author: Patrick Wendell <patrick@databricks.com>

Closes #5757 from pwendell/dataframes and squashes the following commits:

0d931a4 [Patrick Wendell] Attempting to fix PySpark tests
85bf740 [Patrick Wendell] [SPARK-7204] Fix callsite for dataframe operations.