Adds examples to the readme

* adds crawlcount to the crawler (for different rounds of crawling)
Removes unnecessary functions

* Changes values passed into callback fn in retrieve_urls
* moves callback fn higher in the file
* calls retrieve urls in a 'callLater' instead of straight out
Bug Fix! Callback function now gets called (as it should)

* simply changed parameters of the callback fn
Reverts back to calling retrieve_urls directly
Adds recrawling functionality

Bug! Though links are added to the frontier, after the first seeds, the deferred list seems
to return immediately (i.e. the callback is triggered immediately, with no processing messages printed)