Assign Collection in example code to variable
Added Collection.get_element_url() to create an URL for an Element
test_flickr() is now using the localserver plugin

Flickr API keys for testing expire after a short peroid of time. Therefore the
Flickr test is now using the localserver plugin. Only if a Flickr API key has
been defined as FLICKR_API_KEY environment variable the Flickr API will be
used.
Revised Collection.get()

If Resource.id_attribute is defined and present in kwargs use the Element's URL
instead of filtering all Elements.