Add cv::String class
Compile opencv_test_core with cv::String instead of std::string

All tests passed!
All modules (except ocl and gpu) compiles and pass tests
Python wrapper is adapted for cv::String
Java wrapper is adapted for cv::String
Fix build of apps and samples
Fix build of various backends
Use cv::String in Android camera wrapper
Remove all include directives for <string>
Drop cv:: prefix from cv::String used inside the cv namespace
Update documentation
Fix build of ocl module