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
For example Chapter 7.2 of "Modern C++ Design: Generic Programming and Design Patterns Applied" by Andrei Alexandrescu, Addison-Wesley, 2001. [link](http://www.informit.com/articles/article.aspx?p=25264&seqNum=3)