BUG: Add bar kwarg to batch_transform.

Before the change to the RollingPanel, window_length
specified the number of days that should be in a window.

The previous commit broke this if data was minute resolution.

By passing bar='minute' to the batch_transform we internally
multiply the window_length by 60*6.5 to have a full day.

Also adds a (still rudamentary) test for batch_transform
with minute data.