Only colormap adjoined Histogram when dimensions match

[philippjfr]

Fixes dimension lookup error with this example:

%%opts Scatter [color_index=2 size_index=3 scaling_factor=50]
np.random.seed(10)
data = np.random.rand(100,4)

scatter = hv.Scatter(data, vdims=['y', 'z', 'size'])
scatter + scatter[0.3:0.7, 0.3:0.7].hist()

[philippjfr]

Ready to merge, the Points/Scatter examples added to #1476 should cover this case.

[jlstevens]

Thanks for fixing this! Tests are green now, merging.