Restructuring

* got rid of build-time config.hpp, config.hpp is now always there
  build-time flags are passed in through CMake definitions
* all high level copy functions now have src -> dst (STL style)
  backend functions will follow
* numerous functions were not declared inline
* added host_allocator (untested)
* added code do read files from disk
* added pingpoing example
* modified mex example