Extract sourceMappingURL from escaped string

Not sure how it happened, but `getSourceMappingUrl` is operating on an escaped string, so, the newline character is actually `\\n` instead of `\n`. I was able to fix locally by adding `\\` to the regex on this line:

``` js
match = /^[^\r\n\\]+/.exec( substring );
```

but a better fix would be to unescape this string before matching
