Updated test build scripts
Reorganised backend::deleter

This will allow to spread its specifications across relevant files.
Fixed VEX_FUNCTION and VEX_STENCIL macros
Got rid of VEX_GLOBAL_CONSTANT macro
Removed fixed workgroup size constructor from backend::kernel
Documentation updated
Introduced backend::opencl and backend::cuda namespaces

Definitions from either namespace are still visible from vex::backend.
The separation helps to create documentation for both backends.
Mention CUDA backend in README