Drop support for PyQt4 v4.4 (and earlier) by removing old-style signals and slots syntax

[PierreRaybaut]

Following Issue #35, we will remove all the old-style signals and slots.

The following example:

if SIGNAL is None:
    self.widget.valueChanged.connect(layout.update)
else:
    self.connect(self.widget, SIGNAL("valueChanged(int)"), layout.update)

should be replaced by:

self.widget.valueChanged.connect(layout.update)