ndarray API necessary to wrap bolt with xray

[shoyer]

• single argument ufuncs (sin, exp, etc.) and ufunc like functions (pd.isnull, notnull, astype, around, isclose)
• broadcasting binary operations (e.g., ndarray arithmetic)
• three argument version of where (preferably with broadcasting)
• aggregation methods (argmax/argmin/max/min/prod/std/var)
• nan-skipping aggregations (e.g., nanmean, nansum)
• indexing with integer arrays, booleans, slices, None
• transpose
• insert or setting with fancy indexing
• broadcast_to (NumPy 1.10)
• concatenate and stack (NumPy 1.10)

cc @izaid who's interested in this for dynd