Domain info response doesn't properly handle <domain:hostAttr> notation for name server info

The domain info response properly handles name server data that comes in the following shape:

```xml
<domain:ns>
   <domain:hostObj>ns1.example.com</domain:hostObj>
   <domain:hostObj>ns1.example.net</domain:hostObj>
</domain:ns>
```

According the [RFC-5731](http://tools.ietf.org/html/rfc5731#section-1.1) a domain's name servers can be specified in two ways: *"Name servers are specified within a `<domain:ns>` element. This element MUST contain one or more `<domain:hostObj>` elements or one or more `<domain:hostAttr>` elements."*

When `<domain:hostAttr>` is used the name and IP addresses should be specified as follows:

```xml
<domain:ns>
  <domain:hostAttr>
    <domain:hostName>ns1.example.net</domain:hostName>
    <domain:hostAddr ip="v4">192.0.2.2</domain:hostAddr>
    <domain:hostAddr ip="v6">1080:0:0:0:8:800:200C:417A</domain:hostAddr>
  </domain:hostAttr>
</domain:ns>
```

But the code expects `<domain:name>` and `<domain:addr>` nodes instead resulting in a `NoMethodError`:

```
NoMethodError: undefined method `content' for nil:NilClass
    /Users/jacegu/Dev/epp-client/lib/epp-client/domain/info_response.rb:27:in `block (2 levels) in nameservers'
    /Users/jacegu/Dev/epp-client/lib/epp-client/domain/info_response.rb:26:in `each'
    /Users/jacegu/Dev/epp-client/lib/epp-client/domain/info_response.rb:26:in `map'
    /Users/jacegu/Dev/epp-client/lib/epp-client/domain/info_response.rb:26:in `block in nameservers'
    /Users/jacegu/Dev/epp-client/lib/epp-client/domain/info_response.rb:25:in `each'
    /Users/jacegu/Dev/epp-client/lib/epp-client/domain/info_response.rb:25:in `map'
    /Users/jacegu/Dev/epp-client/lib/epp-client/domain/info_response.rb:25:in `nameservers'
    /Users/jacegu/Dev/epp-client/test/domain/test_domain_info_response.rb:118:in `block (3 levels) in <class:TestEppDomainInfoResponse>'
    /Users/jacegu/.rvm/gems/ruby-2.0.0-p481/gems/shoulda-context-1.1.5/lib/shoulda/context/context.rb:400:in `call'
    /Users/jacegu/.rvm/gems/ruby-2.0.0-p481/gems/shoulda-context-1.1.5/lib/shoulda/context/context.rb:400:in `block in create_test_from_should_hash'
```

This PR fixes the parsing of `<domain:hostAttr>` nodes adhering to what's described in the RFC and removing the support for the `<domain:name>` and `<domain:addr>` nodes.

I'm assuming that those nodes were supported by mistake and that are not used but I may be wrong (maybe they come from a previous version of the RFC?). If that's the case I'd happily extend this PR to provide support for both of them.