Add pyqtgraph recipe

There isn't a way right now. We have discussed this before (see https://github.com/conda/conda/issues/793#issuecomment-47788554). At the end of the day, conda has to pick one or the other, so as long as both are available (which they will be for Anaconda users), it will pick the preferred one. It would be different if you preferred pyside, because that isn't available on Python 3 in the Anaconda repos. But if pyqt should be preferred, I would just leave it as is for now.