Fixinf VistaDB assembly discovery

Hi, Demis,
I have fixed and refactoring code towards fixing your problems with loading VistaDB assembly discovery.
Now it is possible to configure the provider either to use a local assembly or one from GAC,
All tests pass now with the assembly in GAC. Please have a look if it works for you.

What about optimistic locking support, VistaDB doesn't support ROWVERSION type and the implementation of triggers  requires additional effort to make this provider feature work. Now VistaDB supports only CLR triggers, which means that we must create an assembly. Here its sample code:

```
VistaDBConnection conn = new VistaDBConnection("Context Connection = true");
// update MyTable Set Version = Version + 1 from MyTable inner join inserted on MyTable.ID = inserted.ID
```

A little bit awkward, but it is the only solution I see by now and if you are not against it I will try to implement it.