default variable name derived from controller

This is a very small change to make the pagination scope default sensibly. The stemming steps are pinched from cancan. 

For a consistent API it means I can just put

```after_filter :paginate, :only => [:index]```

in the base ApiController and then forget about it.

I was tempted to rewrite the interface as `paginate :pages`, but what you have is lovely and simple and I didn't want to spoil that.