API: implement __finalize__ for resample et al.
I'm really salivating at the chance to use the `._metadata` and `__finalize__` mechanisms in pandas-0.13-dev to create my own subclass of DataFrame and to have metadata propagate after calling functions inherited from DataFrame like `dropna()`, `resample()` etc.

Using the latest 0.13-dev version of Pandas, I think I might have bumped into a small bug (although I'm not sure if this is a bug or not??):

`._metadata` propagates after calling `.resample(rule='D')` and `dropna()` but not after calling `.resample(rule='D', how='max')`.

More details, including the full code of my subclass, are given under the "experiments" heading of this issue: https://github.com/nilmtk/nilmtk/issues/83
