compatibility for current ArrayFire and current OpenCV

[meetnick]

With the respect of its creator developer, I send this pull request in order to make this compatible with current versions of ArrayFire and OpenCV (tested on 3.7.0 and 3.4.3, respectively). This pull request includes the following in order to accomplish:

• Stick with std:: library for std::vector and std::string (in order to provide better compatibility and prevent undesired namespace propagation).
• corrects the name of af/utils.h to af/util.h which is how's officially called in ArrayFire.
• updates deprecated colorspace to current colorSpace.
• changes af_cspace_t af_gray and af_rgb to AF_GRAY and AF_RGB, respectivelly (maybe TYPO?)
• Fixes the possibility of crash when using single-dimensional array. On this case, it should consider the number of elements instead ask for nonexistent dim(ndims - 2), which gives segmentation fault error.
• With respect to its creator, I removed "Mat stats" functions definition and declarations. I definitely don't know where _top and _stats came from.

It also solves issue #1

System environment: *buntu 18.04.1 LTS OpenCV 3.4.3 ArrayFire v3.7.0 (CPU|OpenCL, 64-bit Linux, build ef5183d7)

[mcclanahoochie]

niccolatorres thanks for the update!!

[meetnick]

first off, i really appreciate the update!!!

i'd prefer to keep the af:: namespace around, since you didn't remove af:: from af::exception.. so, completely remove af:: everywhere because of the 'using namespace af' , or revert the "af::" changes. thanks again!

I much appreciate to contribute to this project. I indeed agree af:: and even cv:: namespaces should be used explicitly inside to avoid namespace undesirable propagation. I removed af:: from data types because compiler was raising "af:: has no member u8/f64/s32/etc.." Now I fixed it to the correct namespace for data types, which is af::dtype::

Please consider the new commit with these changes requested.