Check browser when setting responsive_viewport size

Some browsers include scrollbar width in their reported inner width, others do not. WordPress core sniffs the user agent to ensure the reported viewport size will be consistent across modern browsers. This patch makes Bones mirror the way core determines viewport size.

See core trac [changeset 24619](http://core.trac.wordpress.org/changeset/24619):
```javascript
// WebKit excludes the width of the vertical scrollbar when applying the CSS "@media screen and (max-width: ...)" 
// and matches $(window).width(). 
// Firefox and IE > 8 include the scrollbar width, so after the jQuery normalization 
// $(window).width() is 884px but window.innerWidth is 900px. 
// (using window.innerWidth also excludes IE < 9) 
respWidth = navigator.userAgent.indexOf('AppleWebKit/') > -1 ? $(window).width() : window.innerWidth; 

if ( respWidth && respWidth < 900 ) { /* ... */ }
```

resolves #468