Add Rails environment to index name

[pfeiffer]

Closes #5

[jeroig]

Hi,

Be careful with this change for two reason.
1- Integration test not pass

1) Failure:
test_index_name(ElasticIndexTest) [./test/define_index_test.rb:10]:
<"users_test"> expected but was
<"elastic_index_test-user">.

2- (and more important yet) Names of the indexes like class names (rails concept)
See the patch fc5a034

I think that you can do the change something like that

@index_name = [options[:index_name] || self.name.underscore.gsub(/\//,'-'), Rails.env].join("_")