Added support to Mexican Hat hash and PCC image comparision

Additional support for:

- int ph_compare_images(const char *file1, const char *file2,double &pcc, double sigma = 3.5, double gamma=1.0, int N=180,double threshold=0.90);
- uint8_t* ph_mh_imagehash(const char *filename, int &N, float alpha=2.0f, float lvl = 1.0f);
- double ph_hammingdistance2(uint8_t *hashA, int lenA, uint8_t *hashB, int lenB);