Add cursor mode to Paginator and DB.

Continuation from #2876. Added `cursor()` method to Eloquent and Fluent.
Works the same way as paginator, except that it'll create Paginator instance in cursor mode (without knowing total number of items available).

Usage with Eloquent:
````php
// Will execute: SELECT * FROM users ORDER BY id LIMIT 0, 31;
$users = User::orderBy('id')->cursor(30);
// Paginator instance $items array will contain 30 items and one extra item would be put into $cursor.
````

And with Paginator:
````php
// On render will return Prev link (disabled) and Next link leading to page 2.
// First argument - items array, second argument - number of items per page. Total will be set to null.
$pager = Paginator::make(['foo', 'bar', 'baz'], 2);
````

P.S.: Removed several stray spaces in tests.