[SPARK-7157][SQL] add sampleBy to DataFrame

[rxin]

This was previously committed but then reverted due to test failures (see #6769).

[SparkQA]

Test build #38868 has finished for PR 7755 at commit 103beb3.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[rxin]

Jenkins, retest this please.

[SparkQA]

Test build #1232 has finished for PR 7755 at commit 103beb3.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[mengxr]

This is different from Scala output because we use different number of threads in python unit tests. See https://issues.apache.org/jira/browse/SPARK-9487.

[SparkQA]

Test build #39094 has finished for PR 7755 at commit 542bd37.

• This patch fails PySpark unit tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • case class DateAdd(startDate: Expression, days: Expression)
  • case class DateSub(startDate: Expression, days: Expression)
  • case class UnixTimestamp(timeExp: Expression, format: Expression)
  • case class FromUnixTime(sec: Expression, format: Expression)
  • case class TimeAdd(start: Expression, interval: Expression)
  • case class TimeSub(start: Expression, interval: Expression)
  • case class AddMonths(startDate: Expression, numMonths: Expression)
  • case class MonthsBetween(date1: Expression, date2: Expression)

[SparkQA]

Test build #39104 has finished for PR 7755 at commit fbf9044.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • case class DateAdd(startDate: Expression, days: Expression)
  • case class DateSub(startDate: Expression, days: Expression)
  • case class TimeAdd(start: Expression, interval: Expression)
  • case class TimeSub(start: Expression, interval: Expression)
  • case class AddMonths(startDate: Expression, numMonths: Expression)
  • case class MonthsBetween(date1: Expression, date2: Expression)

[mengxr]

@rxin Should be ready now.

[rxin]

Merging in master.