Gravatar filter for templates

Your templates can use {{ email|gravatar }} to get the gravatar associated
with the email.
Options (and defaults):
	size=100
	rating='g'
	default='retro'
	force_default=False
	force_lower=False
	use_ssl=False
Gravatar support. Merge from e1a1422dc635195274938c404471fd3f4ed8b307
main_archive uses the same format as archive