This closes #6104.

Author: Cheng Hao <hao.cheng@intel.com>
Author: Reynold Xin <rxin@databricks.com>

Closes #6343 from rxin/window-df and squashes the following commits:

026d587 [Reynold Xin] Address code review feedback.
dc448fe [Reynold Xin] Fixed Hive tests.
9794d9d [Reynold Xin] Moved Java test package.
9331605 [Reynold Xin] Refactored API.
3313e2a [Reynold Xin] Merge pull request #6104 from chenghao-intel/df_window
d625a64 [Cheng Hao] Update the dataframe window API as suggsted
c141fb1 [Cheng Hao] hide all of properties of the WindowFunctionDefinition
3b1865f [Cheng Hao] scaladoc typos
f3fd2d0 [Cheng Hao] polish the unit test
6847825 [Cheng Hao] Add additional analystcs functions
57e3bc0 [Cheng Hao] typos
24a08ec [Cheng Hao] scaladoc
28222ed [Cheng Hao] fix bug of range/row Frame
1d91865 [Cheng Hao] style issue
53f89f2 [Cheng Hao] remove the over from the functions.scala
964c013 [Cheng Hao] add more unit tests and window functions
64e18a7 [Cheng Hao] Add Window Function support for DataFrame