added NSData+SHA256HMAC.h and HMACWithData

Added
 + HMAC with NSData as key to IGHMAC.h
 + new NSData+SHA256HMAC.h category
Now omitting additional encoding from and to cString if key is provided
as raw NSData. As encoding might not be possible (NULL) and will cause
additional effort (__strong const char * of encoded value) until
receiver gets freed or autorelease scope end is reached.