postgres provider update

- remove some unnecessary includes
- add private PQexec method with additional debug output
- update all changed attributes of a feature at once
- fix quoting and beautify creation of SQL statements
- replace assertion on unexpected field types with useful debug output and
    consider values as NULL


git-svn-id: http://svn.osgeo.org/qgis/trunk/qgis@8307 c8812cc2-4d05-0410-92ff-de0c093fc19c

Author: jef