Convert email to lowercase before hashing

See step #2: http://en.gravatar.com/site/implement/hash/
Strip whitespace as per gravatar instructions

http://en.gravatar.com/site/implement/hash/
Use smart_urlencode to encode urls because escape replaces '&' by '&amp;'.
Merge branch 'master' of https://github.com/ipmb/django-gravatar
Added a template tag for generating retina-compatible image tags.
changed user import for django 1.5 compat and used hashlib instead of deprecated hashcompat
Merge remote-tracking branch 'up1/master' into HEAD

Conflicts:
	gravatar/templatetags/gravatar.py
Merge with forks: AGASS007 & Morgul