Use Search::Elasticsearch instead of Elasticsearch, adapt code and tests

Elasticsearch is deprecated since 2014-03-05, and Search::Elasticsearch
replaces it. There were also some API changes which required changes to
the code and tests.
Adapt pod to the change to Search::Elasticsearch
Rename 'servers' to 'nodes' for consistency with Search::Elasticsearch

Also add some code to the BUILDARGS wrapper which emits a warning and
copies the 'servers' parameter to the 'nodes' parameter to stay backwards
compatible (as long as nobody used the 'servers' accessor). As this affects
the model object construction, add a few tests for the configuration
handling, too.
Merge pull request #1 from mstock/feature/search-elasticsearch-migration

Use Search::Elasticsearch instead of deprecated Elasticsearch