Create qetag.js

测试代码：
```JavaScript
var filePath = 'testfile';
getEtag(filePath,function(eTag){
	console.log('filePath',eTag);
});

var fileBuffer = require('fs').readFileSync(filePath);
getEtag(fileBuffer,function(eTag){
	console.log('buffer:',eTag);
});

var fileStream = require('fs').createReadStream(filepath);
getEtag(fileStream,function(eTag){
	console.log('stream:',eTag);
});
```
Merge pull request #12 from TooBug/patch-2

Create qetag.js