remove self from closures in dask.dataframe
remove self from closures in dask.dataframe

Previously the dataframe object contained tasks that closed over the
dataframe itself.  This made certain tasks very costly to serialize.

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