Adding the template struct mextypes 

I have added the struct mextypes. With this class is it easier to get the matlab id of an object with the type. I have Changed the constructor : MxArray::MxArray(const std::vector<T>& v). I have tested it and it return the right values.
Test:

    std::vector<int> test(10);
	test[0]=1;
	test[1]=2;
	test[2]=3;
	test[3]=4;
	test[4]=-5;
    std::vector<double> test2(10);
	test[0]=1;
	test[1]=2;
	test[2]=3;
	test[3]=4;
	test[4]=-5;
	plhs[0] = MxArray(test);
	plhs[1] = MxArray(test);
The old function returned cells. This solution could be usefull for other thins and not just opencv.

I have moved the ClassIDOf and DepthOf maps to the header since I think they could be usefullt elsewhere. If you like this solution you should delete this maps from the cpp file ! I just wannted to pull request on this file.