Initial Python 3 port

Many things work, some don't. Notably, genPOI doesn't work, and
there's some signedness comparison stuff going on in the C extension.

This also completely drops support for Python 2, as maintaining a C
extension for both Python 2 and 3 is a pain and not worth it for the
9 months that Python 2 is still going to be supported upstream.

The documentation needs to be adjusted as well.

All of the few tests we have pass, and rendering a map works, both
with a configuration file and without. We can also use optimizeimages.

Concerns #1528.

overviewer_core/rendermodes.py

@@ -14,7 +14,7 @@
 #    with the Overviewer.  If not, see <http://www.gnu.org/licenses/>.
 
 from PIL import Image
-import textures
+from . import textures
 
 """The contents of this file are imported into the namespace of config files.
 It also defines the render primitive objects, which are used by the C code.
@@ -31,13 +31,13 @@ class RenderPrimitive(object):
             raise RuntimeError("RenderPrimitive cannot be used directly")
         
         self.option_values = {}
-        for key, val in kwargs.iteritems():
+        for key, val in kwargs.items():
             if not key in self.options:
                 raise ValueError("primitive `{0}' has no option `{1}'".format(self.name, key))
             self.option_values[key] = val
         
         # set up defaults
-        for name, (description, default) in self.options.iteritems():
+        for name, (description, default) in self.options.items():
             if not name in self.option_values:
                 self.option_values[name] = default