Fixed error with GAFFERUI_QT_BINDINGS=PyQt4 environment variable.

We weren't correctly applying the fixup for QtCore.Signal in this case - we were only applying it if the environment variable wasn't specified and we decided to use PyQt based on its existence on the filesystem.
Fixed PySide incompatibility in Slider.

PyQt supports the passing of points directly to drawPolygon(), and PySide supports passing them as a list. Neither support the other's syntax. Fortunately they both support the creation of a QPolygonF from a list and then accept that in drawPolygon(). This problem manifested itself when drawing the off-the-end indicator for sliders.
Key modifiers are now correctly updated during drags.

Before this they were stuck with the values they had at the start of the drag.
Constrained slider positions inside range by default.

They can still be dragged outside of the slider range by holding down the Shift key while dragging.

Fixes #99.
Merge pull request #687 from johnhaddon/colorSliderRange

Color slider range constraints