Unification of the upgrade queries

First step and mainly experimental to see if travis complains about anything.

This is something I started last year, and finished only now... I should have worked on it more to finish it for 1.0, right? :stuck_out_tongue: 

The general goal is to merge the upgrade and installation files of the two/three databases supported into a single one.
This PR covers Elk 1.0 upgrade only.
It has also some other goodies, for example I added a Database_Abstract class to collect some code that is shared between MySQL and Postgre (and maybe SQLite) implementations, it also adds a new method to set the "skip error" parameter at "class level" instead of query level.

Once merged fixes #944