Speedy 0.5.1

• Matrix expressions are now compiled for better performance
• Linear Algebra routines are now split into different files
• Introduced SpeedyMatrixExpr.map() and SpeedyMatrixExpr.reduce()
• Introduced SpeedyMatrixExpr.inverse()
• Improved & optimized the LK feature tracker
• ORB: using a larger patch when computing the orientation of a keypoint
• Harris/FAST: using 16-bit half-floats to store the scores of keypoints
• Improved the keypoint encoder
• General improvements & fixes