Support PyQt's "new" build system

PyQt4 ships with a configure.py and configure-ng.py script for
configuration. The former produced the "pyqtconfig" module which was
used to determine various flags required for compilation of
python-poppler-qt4. The configure.py script has been causing some issues
([1]) and upstream suggests to use configure-ng.py instead as the old
configuration script is deprecated ([2]).

This patch adapts setup.py to work without the "pyqtconfig" module by
looking up the required config values via the new mechanisms endorsed by
upstream. Additionally, it adds the --qt-include-dir, --pyqt-sip-dir,
--pyqt-sip-flags command line arguments to the "build_ext" command which
can be used to set those values manually.

[1] http://www.riverbankcomputing.com/pipermail/pyqt/2014-June/034344.html
[2] http://www.riverbankcomputing.com/pipermail/pyqt/2014-June/034354.html
Merge pull request #3 from lorenzph/pyqt-configure-ng-support

Support PyQt's "new" build system