Add urlfetch to download external resources

* Fetch http(s) resources hosted remotely
* Download files chunk by chunk (chunk_size parameter)
* Returns IOError if file byte max_size parameter is exceeded
Note that it will provide a reliable way to check mack_size: it doesn't simply checks Content-Length Header but does the max_size check at each chunk iteration.
* Also work with file:// scheme ("file" has to be specified in allowed_schemes parameters)

A common implementation is used in Heat and Solum, the Solum version was ripped from Heat:
https://github.com/openstack/heat/blob/master/heat/common/urlfetch.py