Speedup for dd.from_dask_array
Speedup for dd.from_dask_array

Previously `range` (or `xrange` for python 2) was used to generate
the index. Now `np.arange` is used instead. This results in a 40x
speedup on my particular problem (for this part), and also releases the
GIL for this step.

mikegraham/dask,jakirkham/dask,jcrist/dask,mraspaud/dask,cowlicks/dask,dask/dask,blaze/dask,cpcloud/dask,mrocklin/dask,ContinuumIO/dask,blaze/dask,mrocklin/dask,gameduell/dask,ContinuumIO/dask,jcrist/dask,mraspaud/dask,dask/dask,chrisbarber/dask,jakirkham/dask