This adds an extents<N> class which stores an array of n-dimensional
extent values. This can be used to represent an ND-range for launching
an OpenCL kernel or for storing the size/shape of a 2D/3D image.
Also adds a variadic dim() function which provides a concise syntax
for creating extents<N> objects.
This adds interoperability support between Boost.Compute and various
other C/C++ libraries (Eigen, OpenCV, OpenGL, Qt and VTK). This eases
development for users using external libraries with Boost.Compute.
