Optional test dependency on pyexcel-handsontable

[jayvdb]

The tests state they depend on pyexcel-handsontable, but that depends on @pyecharts , has problem pyexcel-renderers/pyexcel-handsontable#5 , but generally is less mature than pyexcel.

The test suite should at least skip the relevant tests when pyexcel-handsontable is not installed, so that the packagers dont need to get @pyecharts working before they can get @pyexcel packaged with tests passing.

[chfw]

Let me have a look. I may have winded myself up.