Return callback values in parser to allow synchronous parsing

This is a simple very minimal impact fix for #1498, allowing the parser to be used synchronously when imports do not need to be processed. It also aligns with the convention used in the parser for handling errors, when errors are handled the result of the callback is returned in the current implementation. This fix aligns with that exisitng convention.

This fix allows synchronous execution of the parser using the following code:

```javascript
var parser = new(less.Parser)({
    processImports: false
});
var result = parser.parse(lessSource, function(e, r) {return {error: e, success: r};});
```