Add some explicit register_ptr_to_python calls

[tomhughes]

If mapnik and python-mapnik are built with boost 1.60 then various python-mapnik test fail with errors like there:

TypeError: No to_python (by-value) converter found for C++ type: std::shared_ptr<mapnik::image_any>

There are reports of similar issues at http://permalink.gmane.org/gmane.comp.python.c++/16601 and in the comments of boostorg/python#29 and it seems that the issue is that the shared pointers are being implicitly registered when used in the class definitions, but it seems to be unclear if this was a deliberate change or not.

This patch adds enough register_ptr_to_python calls to get the tests passing, but it's possible that there are others that need to be added that aren't caught be the calls - there are definitely others without explicit registration but it's not clear if the tests just aren't catching them or if they aren't a problem.

[artemp]

Hmm.. just got caught by this issue, looks like we need it for >= boost 1.60 until resolved upstream