Fix issues #5-#9

Fixes #9 - Do not always push rendered html to case, push context if template is specified in decorator.
Fixes #8 - Remove coffin dependency and use internal env while supporting custom jinja2 envs.
Fixes #7 - Use smart_str on rendered html before pushing to cache.
Fixes #6 - Add get_cache function that looks for viewlet backend before fallbacking on default. 
Fixes #5 - If timeout is set to None, set timeout to "infinity" default 1 year