OCL: DFT

(in collaboration with @ElenaGvozdeva).

check_regression=*OCL*Dft*
test_filter=*OCL*Dft*
test_modules=core
build_examples=OFF
Merge after Itseez/opencv#3000

Future work:
- Enabling `double` support
- Minor optimization
I think it is not good idea to return the pointer. Anyway you have to return `const OCLFftPlan *`, but i think will be better use some `smart_ptr` or something like it.