A high-level cross-protocol url-grabber.

Using urlgrabber, data can be fetched in three basic ways:

urlgrab(url) copy the file to the local filesystem
urlopen(url) open the remote file and return a file object (like
urllib2.urlopen)
urlread(url) return the contents of the file as a string

When using these functions (or methods), urlgrabber supports the following
features:

* identical behavior for http://, ftp://, and file:// urls
* http keepalive - faster downloads of many files by using only a single
connection
* byte ranges - fetch only a portion of the file
* reget - for a urlgrab, resume a partial download
* progress meters - the ability to report download progress automatically,
even when using urlopen!
* throttling - restrict bandwidth usage
* retries - automatically retry a download if it fails. The number of retries
and failure types are configurable.
* authenticated server access for http and ftp
* proxy support - support for authenticated http and ftp proxies
* mirror groups - treat a list of mirrors as a single source, automatically
switching mirrors if there is a failure.