[Processing] A new tool to import data in PostGIS based on ogr2ogr and a...

... few other ogr related fixes.

None of the tools available in QGIS (SPIT, DB Manager, drag and drop in Browser, the tool already available in Processing) to allow users import data into PostGIS is very efficient, to tell the truth are pretty slow and they lack many important options (for example the one to append to existing tables). Command line tools are available and rich of options and switches to allow boost importing speed especially of very large vectors.

See 

https://doublebyteblog.wordpress.com/2014/08/06/importing-large-spatial-dataset-into-postgis/comment-page-1/#comment-123

for an example.

While the solution based on shp2pgsql/psql requires to install them, as they are not part of the standard QGIS installation, ogr2ogr is part of any QGIS installation and therefore a Processing tool based on it will be added with this pull request. And the tool is several times faster than any other alternative available in QGIS.