Fixing duplicated go call

https://github.com/shower/shower/issues/191

I've changed `getCurrentSlideNumber` that it return `-1` if there's no slide for requested hash id, to know exactly when client request first slide or non-existent. Then I have updated `popstate` listener, that caused duplicated `shower.go` method call, to run `shower.go` only when it's necessary. 

I have checked this code with `grunt test`, and the first issue with which this part of code was associated (browser back to List mode).