SIFT detector not fully disabled when building for ARM (Bug #772)

Hi!

I've detected this problem while building OpenCV for iOS platform.
The problem is in sift.cpp file in the features2d module.

In the begining of this file there are following lines:
#ifdef arm
#define ARM_NO_SIFT
#endif

And #ifndef ARM_NO_SIFT macro which disables compilation of all SIFT detector code if ARM target detected.
But the declaration of the class still accessible and even more - in cv::FeatureDetector class have static method "create" which can construct detectors by well-known name.

When building OpenCV for iOS i get linker errors of unimplemented SIFT detector class.

Solution:
Wrap SIFT detector class definition and cv::FeatureDetector::create function with ifndef ARM_NO_SIFT to prevent usage this class anywhere if targeting to ARM.