Make JSqueeze filter work with 1.x and 2.x (namespaced)

JSqueeze 2.x just came out, and it's now namespaced. This should let the filter work with both 1.x and 2.x. Obviously it'd also be simple to just make it work with 2.x only if you wanted to go that way.

I thought about adding some helper bits to make it easier to use upstream's default special var regex on 2.x, but thought I'd just keep it simple for now. We could do something like have `setSpecialVarRx` accept 'true', and have that set the value to `JSqueeze::SPECIAL_VAR_PACKER` on 2.x.

@nicolas-grekas wdyt?

It'd be good to have this on the stable branches too.
```
$parser = $this->classname;
$parser = new $parser();
```
?