Let Rails handle defaults for find_in_batches

Rails offers support for custom primary_keys, e.g. UUID.
The start option default always expected an integer PK,
which causes find_in_batches to crash.
Support Sunspot.batch on the sub_session_proxy.
Require Forwardable in sunspot/adapters.
Clean up whitespace
Remove use of deprecated finders

The old finders (from before `ActiveRecord:Relation` style chaining) are
now deprecated in Rails 4. To get ready for the big release let's switch
to the new chaining style to query the database.
Return actual records and not relations

Instead of returning a relation containing the records, let's go ahead
and run the query returning an actual model or array of models.
Correctly ask for a relation object in rails 3

This isn't super pretty but the method used for obtaining a relation has
changed in rails 4.
Invert conditionals for improved readibility

We could use `present?` here instead of `!blank?` but I opted to not
depend on `active_support`.
Merge branch 'master' of github.com:sunspot/sunspot
Merge branch 'master' of github.com:sunspot/sunspot
Make stubbed facet respond to 'rows' with an empty array
Merge remote-tracking branch 'vanstee/fix-rails-4-deprecations'

sunspot/sunspot#360
Merge remote-tracking branch 'jassa/fix-find_in_batches'

sunspot/sunspot#373
Merge remote-tracking branch 'strika/master'

sunspot/sunspot#376
Merge remote-tracking branch 'involver/master'

sunspot/sunspot#375
Merge remote-tracking branch 'gaelmuller/facet-stub'

sunspot/sunspot#439
Remove support for Rails 2.3.
Add changelog entries for fork changes.
Remove the vestiges of rspec 1.* support from sunspot_solr/spec
Update sunspot_rails to use the current version of rspec.