Use Railtie to initialize active support

Using:

```
ruby '2.1.2'
rails '4.1.5'
```

I was getting the following ActionController Exception:
```
undefined method `split_stylesheet_link_tag' for #<#<Class:0x007f2dcc874668>:0x007f2dcc8727a0>
```

I noticed that you were initializing in the Rails::Engine rather than a Rails::Railtie. This PR initializes css_spliter via a Railtie, which resolved my issue.

Thanks.

S