Rename Xray.Element to Xray.Specimen
Rename Xray.Element to Xray.Specimen

We're not always just wrapping a single element, so a slightly more
abstract class name seemed called for.

fukayatsu/xray-rails,mattbrictson/xray-rails,brentd/xray-rails,fukayatsu/xray-rails,brentd/xray-rails,fukayatsu/xray-rails,mattbrictson/xray-rails,fukayatsu/xray-rails,brentd/xray-rails,mattbrictson/xray-rails,mattbrictson/xray-rails,brentd/xray-rails