index_orphans batch_size fix, and deleted_at tweak

* Fixed `batch_size` so the option is respected
* Added support for soft/paranoid deleted objects, something which lots of Rails projects use

Examples against real data, with `batch_size`:

    irb(main):002:0> Job.index_orphans(batch_size: 100000)
    D, [2014-07-21T18:03:58.456963 #27407] DEBUG -- :   SOLR Request (167.9ms)  [ path=select parameters={fq: ["type:Job"], start: 0, rows: 100000, q: "*:*"} ]
    D, [2014-07-21T18:03:58.460621 #27407] DEBUG -- :   SOLR Request (3.1ms)  [ path=select parameters={fq: ["type:Job"], start: 100000, rows: 100000, q: "*:*"} ]
       (2424.6ms)  SELECT id FROM "jobs"
    => []


And with `use_deleted_at`:

    irb(main):003:0> Job.index_orphans(batch_size: 100000, use_deleted_at: true)
    D, [2014-07-21T18:04:14.702333 #27407] DEBUG -- :   SOLR Request (4.9ms)  [ path=select parameters={fq: ["type:Job"], start: 0, rows: 100000, q: "*:*"} ]
    D, [2014-07-21T18:04:14.706563 #27407] DEBUG -- :   SOLR Request (3.2ms)  [ path=select parameters={fq: ["type:Job"], start: 100000, rows: 100000, q: "*:*"} ]
       (2092.9ms)  SELECT id FROM "jobs" WHERE deleted_at IS NULL
    => []