#18 PG type map hard to extended

[Peter Williams]

In dbd_pg, not sure about other dbds, there is no easy
way to extend the type map to support non-standard data
types such as those provided by PostGIS. Currently the
only way, I have found, is to override
DBI::DBD::PG::Database#load_type_map with something
similar to the standard implementation that includes
the new types. This is sub-optimal becuase it would
prevent multiple extension from being used at the same
time and it is prone to breakage on new releases of
this dbd.