Fix PgsqlAdapter native types, default values.

* The native types array were missing 'real','bigint','smallint', 'double precision', 'decimal' and 'numeric'.  See: http://www.postgresql.org/docs/9.1/static/datatype-numeric.html
* Both 0 and the empty string "" are valid defaults but were not being recognized.  Changed to check explicitly for NULL defaults.
phpunit workaround

For some reason, the unit tests don't like the use of NULL here.  Use the
is_null() function instead.
Merge pull request #1 from getloaded/pgsql-number-types

Pgsql number types