AppendableFrameTable -> chunks(pd.DataFrame) sensitive to small chunks
AppendableFrameTable -> chunks(pd.DataFrame) sensitive to small chunks

Two changes to improve user experience on hdfstore format=table stores

1.  Reduce default chunksize from 1e6 to 1e5
2.  Yield small chunk then large chunks to support head() workflows

Fixes https://github.com/ContinuumIO/blaze/issues/986

cc @michaelaye

ContinuumIO/odo,Dannnno/odo,cowlicks/odo,Dannnno/odo,quantopian/odo,ContinuumIO/odo,ywang007/odo,cpcloud/odo,cpcloud/odo,ywang007/odo,blaze/odo,quantopian/odo,alexmojaki/odo,cowlicks/odo,alexmojaki/odo,blaze/odo