Allow deprecate_kwarg to transform arguments
This will allow deprecated kwargs to be transformed before being passed to a function.  For example, in #7963, one could write

``` python

@deprecate_kwarg(old_arg_name='infer_dst', new_arg_name='ambiguous',
                 mapping={True: 'infer', False: 'raise'})
def tz_localize(self, tz, ambiguous=None):
    ...

```

Open issues:
- should we test for a bad mapping at creation time?  (in this PR: yes)
- how much error checking should we do?  (in this PR: pass args unrecognized by a mapping through unchanged; assume a function will do its own error handling)
- should we support both mappings and callables?  (in this PR: yes)
- if so, how should we recognize mappings?  (in this PR: existence of a `get` method)
