API: fillna method argument should be None by default and raise on both ...
...value and method #2027

@wesm what do you think of the API change? If you're ok with it I'll propagate to DataFrame and Panel.

I agree that `method='ffill'` should not be the default. Shouldn't it raise an exception now if you write `ts.fillna()`? Folks who want that should define `TimeSeries.ffill` to forward fill for them (we could even add this convenience method). Go ahead and tweak out the other classes-- let me know when ready for merging
 should be ready to go now.

Added ffill and bfill as convenience functions to Series/DataFrame/Panel
 thank you sir
