datasets loading module

Module for loading datasets for Fall2014 OpenCV Challenge.

I hightly unsure about files structure, because I worked on separate application and just converted it to module today (we need hurry as Challenge should start September, 1).

Document in doc/ is very first version and will be updated.

src/ & include/ contain main code.
samples/ - samples how to work with this code.

tinyxml2/ - 3rd party library, in my project structure was in 3rdparty/ folder, but this didn't work after converting to module.

I only checked that module was successfully built.
please, use "const std::string&" here and in all other similar places.