Moving to DUB and latest DMD

It would be much easily to use cl4d if there is dub support. 

Notes:
* Dub package description. DerelictSDL2 and DerelictGL3 are moved to package dependencies.
* Sources are moved to source subfolder.
* Compilation fixes for dmd v2.065.
* Auto linking to OpenCL libraries.
* Recently I had problems with `CL_INVALID_GL_SHAREGROUP_REFERENCE_KHR` error, that wasn't caught by old cl4d version.
* Examples are moved to new versions of dependencies.
* README is now markdown to handle code snippets.

Tested on Fedora 20 x86_64 and Win7 x86