PostGIS connection is not released when datasource is deleted

[springmeyer]

allows a connection to be released if del postgis_datasource is called

[springmeyer]

Add missing portion of patch, moving PGFinish out of ~Connection () destructor

[springmeyer]

This needs more testing, however for one postgres layer this seems to work fine in either of 3 situations:

1) mapnik process ends 2) postgis datasource object is deleted 3) map containing the datasource is deleted

Illustration of #3:

>>> import mapnik
>>> m = mapnik.Map(45,45)
>>> mapnik.load_map(m,'pg.xml')
SELECT f_geometry_column, srid FROM geometry_columns WHERE f_table_name='world_worldborders' AND f_geometry_column='geometry'
setting geometry field to=geometry
setting SRID to=4326
using srid=4326
using geometry_column=geometry
unknown type_oid=16760
>>> del m
PostGIS: closed connection 0x86cc00
>>> exit()

[springmeyer]

making behavior a user adjustable parameter, allowing for the original, persistent and pooled connection to be maintained

[springmeyer]

applied in r1337