Make `DataFrame.pivot` accepts a list of column names as values

Problem description

Currently DataFrame.pivot accepts either None or a column name for the option values but not a list of column names.

# this works
df.pivot(values = None)
df.pivot(values = 'column_name')

# this doesn't work
df.pivot(values = ['cn1', 'cn2'])

It is not convenient if I want to create a pivot table for multiple columns at the same time. Of course, we can pass None to values and then select columns we want to. But this is not as convenient.

[jorisvandenbossche]

@ibrahimsharaf Did you already go through the contributing guide (to set up development environment and such: http://pandas.pydata.org/pandas-docs/stable/contributing.html). If not, you should start with that.

For this issue: I would recommend to first write some tests for the desired new functionality, and then you can look at changing the pivot function to do this (the function is located here: the pivot function in this file: https://github.com/pandas-dev/pandas/blob/master/pandas/core/reshape/reshape.py)

[sunilk747]

Hello. I am new to this library. I would like to work on this issue. Any sort of help would be appreciated.