DEPR: Deprecate cols in several functions
closes #6680 -- Deprecate `cols` in `drop_duplicates()` and `duplicated()` (rename to `subset`)
closes #6645 -- Deprecate `cols` in `to_csv()` and `to_excel()` (rename to `columns`)
releated to #6686 
Related #5505 -- Deprecate `cols` and `rows` in `pivot_table()` and `crosstab()` (rename to `columns` and `index`)

Related #6581 -- Reminder to come back in 0.16, remove deprecation warnings, and use new arguments

maybe should refactor the code that does this cols deprecation check to a function (and also need to change the ones u did already, eg pivot and such)

maybe stick it in pandas.util.misc or something (their is a decorator deprecate already in pandas.util.decorators but not sure if that would work or maybe it would)
 Made a new decorator that deprecates a keyword argument of a function and implemented on the 6 functions in question.
 Thanks for catching the bone-headed overlook of not using the decorator on to_csv and to_excel
 looks good

@jorisvandenbossche ?
 There should be some wrapping of the function in order to keep its name and docstring.
 wouldn't it make sense to add a version to the decorator, so that this information can be printed as well and maybe make this fail in case some env variable is present (TRAVIS) and pandas version is higher (automatic failures on version update :-) )
 @JanSchulz why would you need a version in this - its already versioned by definition as its in master. 

Even though we have created an issue to take this out for good (say in 0.16), that might get changed until its actually removed. Their are some very old deprecations thatreally do need to get taken out. 
 I mean something like this: `deprecate_kwarg(old_arg_name="old", new_arg_name="new", version="0.5")` will output "old is deprecated, use new instead (old will be removed in version 0.5)" and tests using old will fail on travis if pd.**version** is higher than 0.5.
 can't do it because what happens when that 0.5 changes to 0.6 (eg we didn't remove it when we said but later)

it's not always clear how long to deprecate and when to remove

in theory you are right but practically it's a problem
 Added wraps decorator
 merged via https://github.com/pydata/pandas/commit/599156fe06ca7f4bc7f87fca982f561f0ec675a9

thanks!
