Fix issue with tab character in String literal

Hi,

There is an issue in the current JSMin implementation, which doesn't handle properly tab character in String literal.

For instance, if you try to minify the following code, the tab characters will be modified in the String literal, when it should not.

```javascript
function myFunction() {		
    console.log("	my message with tabs\
	in multiline"); 
}
```
This should be minified as 

```javascript
function myFunction() {console.log("	my message with tabs\
	in multiline");}
```

And currently, it is minified like this : 

```javascript
function myFunction() {console.log(" my message with tabs\
 in multiline");}
```

Cheers,
PS : Thanks a lot for your amazing work in the JS world :-)