It is useful to let the user decide the number of rows to show in DataFrame.show

Author: Jacky Li <jacky.likun@huawei.com>

Closes #4767 from jackylk/show and squashes the following commits:

a0e0f4b [Jacky Li] fix testcase
7cdbe91 [Jacky Li] modify according to comment
bb54537 [Jacky Li] for Java compatibility
d7acc18 [Jacky Li] modify according to comments
981be52 [Jacky Li] add numRows param in DataFrame.show()