Also moved all the deprecated functions into one place for SQLContext and DataFrame, and updated tests to use the new API.

Author: Reynold Xin <rxin@databricks.com>

Closes #6210 from rxin/df-writer-reader-jdbc and squashes the following commits:

7465c2c [Reynold Xin] Fixed unit test.
118e609 [Reynold Xin] Updated tests.
3441b57 [Reynold Xin] Updated javadoc.
13cdd1c [Reynold Xin] [SPARK-7654][SQL] Move JDBC into DataFrame's reader/writer interface.