The crawl utility starts a depth-first traversal of the web at the
specified URLs. It stores all JPEG images that match the configured
constraints.  Crawl is fairly fast and allows for graceful termination.
After terminating crawl, it is possible to restart it at exactly
the same spot where it was terminated. Crawl keeps a persistent
database that allows multiple crawls without revisiting sites.

The main reason for writing crawl was the lack of simple open source
web crawlers. Crawl is only a few thousand lines of code and fairly
easy to debug and customize.

Some of the main features:
 - Saves encountered JPEG images
 - Image selection based on regular expressions and size contrainsts
 - Resume previous crawl after graceful termination
 - Persistent database of visited URLs
 - Very small and efficient code
 - Supports robots.txt
