[STORE] Added, that `find_in_batches` returns an Enumerator when the block is not passed

Example:

    Person
      .find_in_batches(size: 100)
      .each { |batch| puts batch.results.map(&:name) }
    # => Test 0
         Test 1
         Test 2
         Test 3
         Test 4

See:

* http://ruby-doc.org/core-2.1.2/Object.html#method-i-to_enum
* http://blog.arkency.com/2014/01/ruby-to-enum-for-enumerator/