[SPARK-10079] [SPARKR] Make 'column' and 'col' functions be S4 functions.

1.  Add a "col" function into DataFrame.
2.  Move the current "col" function in Column.R to functions.R, convert it to S4 function.
3.  Add a s4 "column" function in functions.R.
4.  Convert the "column" function in Column.R to S4 function. This is for private use.

Author: Sun Rui <rui.sun@intel.com>

Closes #8864 from sun-rui/SPARK-10079.