Map and Layer objects both have a new parameter 'srs', initialized to "+proj=latlong +datum=WGS84" by default.
Basic usage (Python):
p = Projection("+proj=merc +datum=WGS84")
point = p.forward(Coord(-2,51))
...
2.reflected arithmetic operators for Envelope/Coord into Python
3.altered return policies for python objects
4.modified build system to require proj4 lib and headers
