Fix issue with parsing of Tweets

I was using this gem and I kept getting #11 . I tracked it down to tweets which have a stucture like:

```xml
<message>
  <id>111</id>
  <tweet>
    <id>222</id>
  </tweet>
</message>
```

The following code concatenates both the ```<id>``` elements, and causes message parsing to fail.

```ruby
@id = message.css('id').text #returns "111222"
```

I changed to use xpath instead of css selectors through the lib, and added an rspec test to catch this particular case.