Add `index` param to `to_dict` like in `to_json`.

[pavelkomarov]

df.to_json(orient='split', index=False) is really nice. df.to_dict(orient='split') basically does the same thing, but I get a dictionary back instead of a string. For symmetry, I'd like to_dict to also have an index param, so I can keep from having to del dictionary['index'] to make them really similar.

[vruthik]

take

[rhshadrach]

but I get a dictionary back instead of a string

I'm not sure I follow what the expectation is here. But in any case, I'm +1 in adding index for API consistency with other DataFrame.to_* methods.