Drop head classes in favour of more reusable functions

* Drop the RoboHydraHead* custom classes
* Make instead a function that replaces the "handler" of a given
  head so it only processes requests for a given Flickr API method,
  and a function that produces the XML for an array of photos.
Minor test improvements

* Really return one result, not two, in the oneSearchResult
  scenario.
* Add instructions to the oneSearchResult scenario so it's clear
  what to do to perform the test.
* Create a function "instructions" to avoid repeating the
  preparation step text (running clj-flickr-memories with
  appropriate parameters).
Add more test scenarios
Catch exceptions when searching for photos

For now, just ignore the result and keep going as if there were
no search results.