Add method to allow for loading of multiple stylesheets

`appendStylesheet:` method allows user to add or override styles defined in the initial stylesheet. This allows you to do things like:

```
[NUISettings initWithStylesheet:@"Base"];
if (SYSTEM_VERSION_LESS_THAN(@"7.0")) {
    [NUISettings appendStylesheet:@"iOS6"];
}
```

where the styles defined in the `iOS6` stylesheet above will add to or override those specified in the `Base` stylesheet (assuming you had a `#define` for `SYSTEM_VERSION_LESS_THAN`).