Adjust set_namespace to default to using meta if undefined.

Can be set to namespace string, empty string or false. This patch stemmed from this [commit thread](https://github.com/zurb/foundation/commit/ed4b8fbd9c61f56e06429ee44f9ac433db8c5c96#commitcomment-5840347).

The update better supports overrides via JS, maintaining backward compatibility. It will always default to reading the value out of the ```<meta>``` tag. If the meta font-family value is 'false', the global namespace value will default to an empty string. It will continue to allow developers to override the namespace via JS.

Override options are:

Custom value:
```Foundation.global.namespace = 'custom-namespace';```

Blank (empty string)
```Foundation.global.namespace = '';```

or False (Boolean or String), will default to empty string
```Foundation.global.namespace = false;```

For example, the following block:
```
Foundation.global.namespace = 'my-app';
$(document).foundation();
```
Will namespace components: ```data-my-app-<component>```