[SPARK-8186] [SPARK-8187] [SPARK-8194] [SPARK-8198] [SPARK-9133] [SPARK-9290]...

[SPARK-8186] [SPARK-8187] [SPARK-8194] [SPARK-8198] [SPARK-9133] [SPARK-9290] [SQL] functions: date_add, date_sub, add_months, months_between, time-interval calculation

This PR is based on #7589 , thanks to adrian-wang

Added SQL function date_add, date_sub, add_months, month_between, also add a rule for
add/subtract of date/timestamp and interval.

Closes #7589

cc rxin

Author: Daoyuan Wang <daoyuan.wang@intel.com>
Author: Davies Liu <davies@databricks.com>

Closes #7754 from davies/date_add and squashes the following commits:

e8c633a [Davies Liu] Merge branch 'master' of github.com:apache/spark into date_add
9e8e085 [Davies Liu] Merge branch 'master' of github.com:apache/spark into date_add
6224ce4 [Davies Liu] fix conclict
bd18cd4 [Davies Liu] Merge branch 'master' of github.com:apache/spark into date_add
e47ff2c [Davies Liu] add python api, fix date functions
01943d0 [Davies Liu] Merge branch 'master' into date_add
522e91a [Daoyuan Wang] fix
e8a639a [Daoyuan Wang] fix
42df486 [Daoyuan Wang] fix style
87c4b77 [Daoyuan Wang] function add_months, months_between and some fixes
1a68e03 [Daoyuan Wang] poc of time interval calculation
c506661 [Daoyuan Wang] function date_add , date_sub