Add support for fl parameter to define which fields should be returned

**Description**
With this pull request it is possible to define the fields which should be returned in the response, e.g `:field_names => [:id, :title]`

**Example request**
```ruby
require 'rsolr-ext'
solr = RSolr::Ext.connect
solr_params = {
  :page=>2,
  :per_page=>10,
  :field_names => [:id, :title]
  :queries=>'ipod',
}
response = rsolr.find(solr_params)
```

**Remark to rcov**
I have dropped *rcov* because it is not supported for ruby >= 1.9. Maybe it would be a good idea to use SimpleCov instead: https://www.ruby-toolbox.com/projects/simplecov