ENH: an example of a sandbox feature

Earlier today I suggested allowing "sandbox" features  into releases,
to make it easier to get some more testing and feedback before
commiting something into core proper.

Here's an example of how I imaging this to work:

``` 
In [1]: pd.describe_option("sandbox")
sandbox.experimental_feature: [default: False] [currently: False]
: boolean
        Enables a snazzy, but not yet stable feature that could use
        some feedback before going into core.

In [1]: pd.options.sandbox.experimental_feature=True

This is an experimental feature being considered for inclusion in pandas core.
We'd appreciate your feedback on it in the appropriate Github issue page:

    http://github.com/pydata/pandas/issues/12345

If you find this feature useful, lacking in major functionality or buggy please
take a moment to let us know, so we can make pandas (even) better.

Thank you,

The Pandas dev team

P.S.

love the hair.

In [2]: df=mkdf(3,2)
   ...: df.foo()
Hi, I'm a new experimental method. grrrr!

In [3]: pd.options.sandbox.experimental_feature=False

In [4]: df=mkdf(3,2)
   ...: df.foo()
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-4-66a4f195a54c> in <module>()
      1 df=mkdf(3,2)
----> 2 df.foo()

/home/user1/src/pandas/pandas/core/frame.pyc in __getattr__(self, name)
   1965             return self[name]
   1966         raise AttributeError("'%s' object has no attribute '%s'" %
-> 1967                              (type(self).__name__, name))
   1968 
   1969     def __setattr__(self, name, value):

AttributeError: 'DataFrame' object has no attribute 'foo'

```