ENH: If a ufunc reduces frame to 0-d, return scalar. Needed for ndimage.

Happy Saturday. I weirdly couldn't sleep past the daylight, so I threw together a [proof-of-concept example notebook](http://nbviewer.ipython.org/github/danielballan/trackpy/blob/parallel/examples/parallel-locate.ipynb) using IPython.parallel and locate. It requires this fix before I merge.

Before:

    In [4]: image.sum()
    Out[4]: Frame(34423137L, dtype=uint64)

    In [5]: ndimage.center_of_mass(image)
    TypeError: 'numpy.float64' object is not iterable

After:

    In [4]: image.sum()
    Out[4]: 34423137

    In [5]: ndimage.center_of_mass(image)
    Out[5]: (211.53404859644257, 318.25055525880748)

I couldn't find any implementations of this kind of scalar reduction online, so I invented one that seems reasonable. (In this case pandas Series v < 0.13 is not analogous: all the ufuncs were rewritten to be index- and NA-aware.)

Also, in a gesture of diligence that eluded me on Friday afternoon, all trackpy tests pass using this pims branch.