Faster import of PyQtGraph.

* RawImageWidget (and thus OpenGL) isn't imported by default anymore.
* scipy.stats.scoreatpercentile is replaced by numpy.percentile.

This commit has not been tested as the example runner is currently
broken.
Merge pull request #11 from anntzer/fast-import

Faster import of PyQtGraph.
Avoid calling QGraphicsWidget.itemChange--this causes segfault in python3 + pyqt

Fixes #10