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

[changhiskhan]

...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.

[wesm]

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

[changhiskhan]

should be ready to go now.

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

[wesm]

thank you sir