Tweak broadcast_variables
Expand _broadcast_compatible_variables to take any number args
broadcast_variables handles any number of args
Update broadcast_variables docstrings
vastly simplify Dataset.to_dataframe() by using broadcast_variables
Fix DataArray.to_dataframe() index names order

Fixes #260.
Add broadcast_arrays
Merge pull request #261 from shoyer/broadcast

Improved broadcasting and new broadcast_arrays function