Made page parameter changeable in Pagination class

The pagination page get parameter can now be changed in the environment,
which will set it for all subsequently created instances of the
paginator.

This is probably not the best way of doing it, but it is the best way that I could think of without making major changes to the structure of the Pagination package.

Allows you set it like so:
```php
Paginator::setPageParam('foo');
$p = DB::table('sometable')->paginate(); // pagination instance has page parameter set as 'foo'
```