Add `crosstab` to SparkR DataFrames, which takes two column names and returns a local R data.frame. This is similar to `table` in R. However, `table` in SparkR is used for loading SQL tables as DataFrames. The return type is data.frame instead table for `crosstab` to be compatible with Scala/Python.

I couldn't run R tests successfully on my local. Many unit tests failed. So let's try Jenkins.

Author: Xiangrui Meng <meng@databricks.com>

Closes #7318 from mengxr/SPARK-8364 and squashes the following commits:

d75e894 [Xiangrui Meng] fix tests
53f6ddd [Xiangrui Meng] fix tests
f1348d6 [Xiangrui Meng] update test
47cb088 [Xiangrui Meng] Merge remote-tracking branch 'apache/master' into SPARK-8364
5621262 [Xiangrui Meng] first version without test