Add append/prepare/overwrite option to PostGIS gui

[robe]

I just noticed the c|a|d|p options are missing in the gui. So I assume it always does a create?

This is a feature I personally use often, so would be nice to have it.

(c|a|d|p) These are mutually exclusive options: I guess we can make it a drop down or radio (though typing it in for now would work fine and defaulting it to c)

But perhaps its too difficult to implement for this first go around so will tag for future.