Pointer typedefs for simple geometry and urdf::*_pointer_cast function

[scpeters]

I had forgotten to include the Box, Cylinder, Mesh, and Sphere classes in the list of pointer typedefs in #13 . Since the lists were getting long, I followed @isucan 's suggestion and created a macro for defining the typedef's (13bf971). It was then easy to add the simple geometry classes (d37907c).

I also added urdf::dynamic_pointer_cast and several other pointer casting functions to further simplify migration. This is important because urdfdom and sdformat do pointer casts from Geometry to derived types:

• https://github.com/ros/urdfdom/blob/ccc7d2ff6653e190907e5023379b757784a36cea/urdf_parser/src/link.cpp#L549-L563
• https://bitbucket.org/osrf/sdformat/src/fa037882e18eabfc5676ec8fb25858d94aa308a3/src/parser_urdf.cc?fileviewer=file-view-default#cl-1744

[scpeters]

I also removed an unused boost header include in b8be5cd