Add tests for nunique on series and on group by
Add tests for nunique on series and on group by

simudream/dask,pombredanne/dask,chrisbarber/dask,freeman-lab/dask,jayhetee/dask,dask/dask,mraspaud/dask,mrocklin/dask,mikegraham/dask,jakirkham/dask,blaze/dask,vikhyat/dask,PhE/dask,mraspaud/dask,gameduell/dask,mrocklin/dask,blaze/dask,pombredanne/dask,clarkfitzg/dask,cpcloud/dask,freeman-lab/dask,ssanderson/dask,dask/dask,ssanderson/dask,clarkfitzg/dask,cowlicks/dask,ContinuumIO/dask,ContinuumIO/dask,jakirkham/dask,simudream/dask,vikhyat/dask,jcrist/dask,jcrist/dask,PhE/dask,wiso/dask,jayhetee/dask,wiso/dask