Added Structual SIMilarity calculation in a JS file. Used for checking
SSIM between contiguous frames in a near-frozen frames subtest of
camtest.js.
Changed statement on where SSIM comes from and what we use from its Matlab code.
Merge pull request #154 from GoogleChrome/cameratest_detect_frozen_frames

Added frozen frames detection in test-rtc based on Structual SIMilarity calculation.