Adds the #find_xpath alias to Capybara::Email::Node

- Capybara 2.1 uses #find_xpath to implement #has_link?
  and other matchers. Since #find uses xpath exclusively,
  this adds an alias for #find as #find_xpath
Merge pull request #31 from pivotaldayjam/find_xpath

Adds the #find_xpath alias to Capybara::Email::Node