Added a groupby argument to the histogram operation

[philippjfr]

As suggested in #1716 we should have an easier way to generate a grouped/faceted histogram. The current approach is as follows:

hv.Dataset(autompg).groupby('cyl').hist(dimension='mpg', adjoin=False).overlay()

For simplicities sake I suggest adding this:

hv.Dataset(autompg).hist(dimension='mpg', groupby='cyl', adjoin=False)

OR

histogram(hv.Dataset(autompg), dimension='mpg', groupby='cyl')

Going to add gallery demos before this is ready to merge.

[jlstevens]

Is a nice exception raised if you try to groupby dimensions that are not available/make no sense?

[philippjfr]

Just the usual exception raised when a dimension is not found:

KeyError: u'Dimension cylinder not found'

[jlstevens]

Maybe groupby should be explicitly listed as an argument of .hist so you can see this argument exists when inspecting the signature (instead of just seeing **kwargs).

[philippjfr]

Let's merge this now, you've already opened an issue to make the .hist signatures more consistent.

[jlstevens]

Sounds good.