[SPARK-8995] [SQL] cast date strings like '2015-01-01 12:15:31' to date

Jira https://issues.apache.org/jira/browse/SPARK-8995

In PR #6981we noticed that we cannot cast date strings that contains a time, like '2015-03-18 12:39:40' to date. Besides it's not possible to cast a string like '18:03:20' to a timestamp.

If a time is passed without a date, today is inferred as date.

Author: Tarek Auel <tarek.auel@googlemail.com>
Author: Tarek Auel <tarek.auel@gmail.com>

Closes #7353 from tarekauel/SPARK-8995 and squashes the following commits:

14f333b [Tarek Auel] [SPARK-8995] added tests for daylight saving time
ca1ae69 [Tarek Auel] [SPARK-8995] style fix
d20b8b4 [Tarek Auel] [SPARK-8995] bug fix: distinguish between 0 and null
ef05753 [Tarek Auel] [SPARK-8995] added check for year >= 1000
01c9ff3 [Tarek Auel] [SPARK-8995] support for time strings
34ec573 [Tarek Auel] fixed style
71622c0 [Tarek Auel] improved timestamp and date parsing
0e30c0a [Tarek Auel] Hive compatibility
cfbaed7 [Tarek Auel] fixed wrong checks
71f89c1 [Tarek Auel] [SPARK-8995] minor style fix
f7452fa [Tarek Auel] [SPARK-8995] removed old timestamp parsing
30e5aec [Tarek Auel] [SPARK-8995] date and timestamp cast
c1083fb [Tarek Auel] [SPARK-8995] cast date strings like '2015-01-01 12:15:31' to date or timestamp