[SPARK-3047] [PySpark] add an option to use str in textFileRDD

str is much efficient than unicode (both CPU and memory), it'e better to use str in textFileRDD. In order to keep compatibility, use unicode by default. (Maybe change it in the future).

use_unicode=True:

daviesliudm:~/work/spark$ time python wc.py
(u'./universe/spark/sql/core/target/java/org/apache/spark/sql/execution/ExplainCommand$.java', 7776)

real	2m8.298s
user	0m0.185s
sys	0m0.064s

use_unicode=False

daviesliudm:~/work/spark$ time python wc.py
('./universe/spark/sql/core/target/java/org/apache/spark/sql/execution/ExplainCommand$.java', 7776)

real	1m26.402s
user	0m0.182s
sys	0m0.062s

We can see that it got 32% improvement!

Author: Davies Liu <davies.liu@gmail.com>

Closes #1951 from davies/unicode and squashes the following commits:

8352d57 [Davies Liu] update version number
a286f2f [Davies Liu] rollback loads()
85246e5 [Davies Liu] add docs for use_unicode
a0295e1 [Davies Liu] add an option to use str in textFile()
[SPARK-2917] [SQL] Avoid table creation in logical plan analyzing for CTAS

Author: Cheng Hao <hao.cheng@intel.com>

Closes #1846 from chenghao-intel/ctas and squashes the following commits:

56a0578 [Cheng Hao] remove the unused imports
9a57abc [Cheng Hao] Avoid table creation in logical plan analyzing