[ENH] Add orient argument and split option to to_dict. (GH7840)
closes #7840 

Unlike the implementation offered in #7840, this version preserves the option to pass prefixes to the `orient` parameter.

I also rewrote the docstring to use the more readable list format from `pd.DataFrame.to_json`.

@rpetchler ok!

pls add a release note in v0.15.0 (in the deprection section) and we can get this in.
 pls squash: https://github.com/pydata/pandas/wiki/Using-Git

@jorisvandenbossche any other comments?
 Hmm... squashed the commits but accidentally merged as well. Advice on how to undo all this?

Thanks for your patience--
 `git rebase -i origin/master`  then delete all the uneeded commits, and f/s the others
 For the rest looking good!
 merged via https://github.com/pydata/pandas/commit/5995dc36a090c193bca20d18f7089503a8674bbb

@rpetchler I squashed and merged it. thanks!
