Create openblas_get_parallel to retrieve information about the used threading model

I integrated a function:

    int openblas_get_parallel(void); 

to detect at runtime which parallelization technique is used by OpenBLAS. In addition I a three constants to cblas.h to identify the return value: 

    #define OPENBLAS_SEQUENTIAL  0  - means that OpenBLAS operates in sequential mode   
    #define OPENBLAS_THREAD  1    - means that OpenBLAS uses the normal threading code 
    #define OPENBLAS_OPENMP 2    - means that OpenBLAS uses the OpemMP threading code.

I hope that this simple function is useful for other users of OpenBLAS too.