qgis: update to 2.4.0

update requirements to use build_env so they can find python packages
internal qwtpolar not compatible with qwt 6.1, use external qwtpolar
add explicit dependencies: sqlite
add new dependencies: numpy, qscintilla2
docs only require postgresql to access postgis servers, remove postgis dependency
removed PyQtImportable message reference to non-existient pyqt info
use Python.site_packages where possible