improved firebird support

I've submitted a patch to improve Firebird support. It runs best
against a Firebird 2.0 database since rails uses certain queries with
COLUMN = NULL in the where clause that won't work in Firebird 1.5

If anyone here uses Firebird it would be great to try it out/review it
and put some comments on it. You'll want to make sure to install the
latest FireRuby driver from ruby forge.

Scott Knight