Expose proj_transform in python bindings

[artemp]

The forward and inverse methods in the python bindings do not seem to allow for transformation between two projected systems, only to and from wgs 84 geographic coordinates.

It would be really useful if the proj_transform(source, destination) function in:
source:trunk/src/proj_transform.cpp
could be exposed in:
source:trunk/bindings/python/mapnik_projection.cpp
or perhaps:
source:trunk/bindings/python/mapnik_proj_transform.cpp

[artemp]

[artem] I agree this would be useful .

[artemp]

[springmeyer] r792 added support for the ProjTransform class in mapnik python, however the code support only Coord objects.

Reopening until we add support for Envelopes.

[artemp]

[springmeyer] r822 adds support for Envelope transformation.