Removing sniffing in Fx.Reveal, more info in the PR

Removed:
```javascript
!Browser.ie6
```
Substituted with:
```javascript
'opacity' in document.documentElement
```
I realise that this is not the same since `opacity` will not be available until ie9, but even so, later hideinputs will consider all ie version, I believe the correct behaviour will be this one, fine to change it if you provide a better solution.