Found a bug in garden.js

Sorry I don't know how to do a pull-request, but I think I found a bug in garden.js...
In the `mainNav.slideUp` event handler: `this` has no `removeAttr` function.

On line 187:
was:  

``` javascript
    mainNav.slideUp(300, function() {this.removeAttr('style');});
```

fixed: 

``` javascript
    mainNav.slideUp(300, function() {$('#nav_main').removeAttr('style');});
```
