Added a comparison operator for TabRow objects (untested)
Added a convenience function to access rows by a single column/value-pair
Fixed a bug that lead to looking up stale DbTab references after opening a new database.

Added a few (untested) convenience lookup-functions to DbTab.
Introduced a "backdoor" in TabRow for accessing the database object
Various improvements:
  * support for deleting rows
  * convience functions for easy search based on a single column value
  * fixed a flaw that prevented proper foreign key clauses for more than one foreign key in a table
  * switched all internal passing of database refs to pointers