Added support for Cpanel::JSON::XS

This PR adds support for specifying a particular backend:

    $e = Search::Elasticsearch->new( serializer => 'JSON::Cpanel');      # Cpanel::JSON::XS
    $e = Search::Elasticsearch->new( serializer => 'JSON::XS');          # JSON::XS
    $e = Search::Elasticsearch->new( serializer => 'JSON::PP');          # JSON::PP

The default module will:

* check if Cpanel is already loaded, and if so, use the Cpanel backend
* else check if JSON::XS is already loaded, and if so, use the JSON::XS backend
* else try to load Cpanel and use the Cpanel backend
* else try to load JSON::XS and use the JSON::XS backend
* else try to load JSON::PP and use the JSON::PP backend
* or die

Fixes #25