Organise tests into test suite per method
DRY up AddTest
Drop unused parameter from with_rummager_service_name
Port tests for Rummageable.index to Rummageable::Index

Rummageable::Index gives you an index object that you can use to add
single documents (with `Index#add`) or multiple documents (with
`Index#add_batch`). Call `#add` when you've just got one document to add
to the index and need the call to run quickly (such as when you're
handling an HTTP request). Call `#add_batch` when you have lots of
documents to index and can cope with a slow/blocking API call (e.g. in a
Rake task).
Add tests for Rummageable::Index's error handling

Instantiate the Index object with `:timeout` and `:attempts` options to
control how many times a connection to Rummager should be retried on
failure.
Add delete and delete_all to Rummageable::Index
Add Rummageable::Index#commit
Add Rummageable::Index#amend
Remove old API and implementation
Add Rummageable::FakeIndex for use in dev/test
Remove dependency on Plek

It makes more sense to allow code that uses Rummageable::Index to
determine how to find the elasticsearch server; it's just config, and
there's no need to couple ourselves to Plek inside Rummageable.