Changed SRID from text to integer type for PostGIS in AddGeometryColumn statement

[copierrj]

PostGIS 2 requires this parameter to be an integer. For PostGIS 1.5 the
value should be -1 when the srid is unknown. In PostGIS 2 providing a negative
value for srid is equivalent to providing 0, which is the PostGIS 2 value
for an unknown srid.

PostGIS versions tested:

• 1.5
• 2.1

Test performed:

create table test(id serial);
select AddGeometryColumn('public', 'test', 'geometry', -1, 'GEOMETRY', '2');
select srid from public.geometry_columns where f_table_name = 'test';

Test result:

• 1.5: srid = -1
• 2.1: srid = 0

Todo: also test in 2.0

fixes #531