fix _repr_html_
label() without changing anything is a noop
merge accepts kwargs

cc @JosephCottam
fix repr_html test
restrict top-level blaze.expr api
restrict public facing blaze.data api
clean up some __all__ definitions
don't dump chunks into main namespace
Data passes kwargs to resource
add test to Data-resource kwarg change

cc @quasiben @chdoig
clean up header handling in csv
Merge pull request #760 from mrocklin/api-play

Api play