add unit/integrted tests for dpc

Yes, I copied the citation from Google Scholar. I will double check the citing formats.

> On Dec 1, 2014, at 3:07 PM, Thomas A Caswell <notifications@github.com> wrote:
> 
> In skxray/dpc.py:
> 
> > +        Weighting parameter (valid range is [0, 1]) for the phase gradient 
> > +        along x and y direction when constructing the final phase image.
> > +        Default value = 0.5, which means that gx and gy equally contribute to
> > +        the final phase image.
> > +
> > +    Returns
> > +    -------
> > +    phi : 2-D numpy array
> > +        Final phase image.
> > +
> > +    References
> > +    ----------
> > +    [1] Yan, Hanfei, Yong S. Chu, Jorg Maser, Evgeny Nazaretski, Jungdae Kim,
> > +    Hyon Chol Kang, Jeffrey J. Lombardo, and Wilson KS Chiu, "Quantitative
> > +    x-ray phase imaging at the nanoscale by multilayer Laue lenses," Scientific
> > +    reports 3 (2013).
> Should 'Reports' be capitalized?
> 
> —
> Reply to this email directly or view it on GitHub <https://github.com/Nikea/scikit-xray/pull/177/files#r21115528>.
> 

--
Best Regards,

  Cheng Chang 
  Computer Engineering, Stony Brook University
  Computer Science Group, Computational Science Center, Brookhaven National Laboratory
  Tel：(001) 631-885-4922