GImage: Replace cv::Mat with A Tiny Header

GImage is a header only implementation of "cv::Mat", and the main implementation is inspired by opencv 2.4.9.

But there are some modification:

• Added support of different alignment such as 256bit for SIMD acceleration (Used by GSLAM Vocabulary)
• Supported no copy convertion from other Mat versions including opencv 3.1, 3.2

Example Usages:

cv::Mat mat(rows,cols,type);
GSLAM::GImage  img=mat;// obtain GImage without memory copy and it will release the buffer correctly

// most functions provided by GImage are the same with cv::Mat
EXPECT_TRUE(img.cols==mat.cols);
EXPECT_TRUE(img.rows==mat.rows);
EXPECT_TRUE(img.empty()==mat.empty());
EXPECT_TRUE(img.type()==mat.type());
EXPECT_TRUE(img.channels()==mat.channels());

EXPECT_TRUE(img.elemSize()==mat.elemSize());
EXPECT_TRUE(img.elemSize1()==mat.elemSize1());
EXPECT_TRUE(img.total()==mat.total());