跨域获取资源，以及YouTube视频封面地址
在HTML中跨域获取资源没有问题，但是在JS中获取跨域资源则不行。如何解决这个问题，OS上有个详细解释：No 'Access-Control-Allow-Origin' header is present on the requested resource—when trying to get data from a REST API
YouTube Video Thumbnails
Each YouTube video has 4 generated images. They are predictably formatted as follows:
http://img.youtube.com/vi/<insert-youtube-video-id-here>/0.jpg
http://img.youtube.com/vi/<insert-youtube-video-id-here>/1.jpg
http://img.youtube.com/vi/<insert-youtube-video-id-here>/2.jpg
http://img.youtube.com/vi/<insert-youtube-video-id-here>/3.jpg
The first one in the list is a full size image and others are thumbnail images. The default thumbnail image (ie. one of 1.jpg, 2.jpg, 3.jpg) is:
http://img.youtube.com/vi/<insert-youtube-video-id-here>/default.jpg
For the high quality version of the thumbnail use a url similar to this:
http://img.youtube.com/vi/<insert-youtube-video-id-here>/hqdefault.jpg
There is also a medium quality version of the thumbnail, using a url similar to the HQ:
http://img.youtube.com/vi/<insert-youtube-video-id-here>/mqdefault.jpg
For the standard definition version of the thumbnail, use a url similar to this:
http://img.youtube.com/vi/<insert-youtube-video-id-here>/sddefault.jpg
For the maximum resolution version of the thumbnail use a url similar to this:
http://img.youtube.com/vi/<insert-youtube-video-id-here>/maxresdefault.jpg
图片尺寸：
0-480x360
1-120x90
2-120x90
3-120x90
mqdefault-320x180
hqdefault-480x360
sddefault-640x480
maxresdefault-1280x720
default-120x90