Fixed incorrect method call in cache.js

Cache::get was calling simpleStorage instead of simpleStorage.get()
Signed-off-by: Gideon Farrell <me@gideonfarrell.co.uk>
Added the rest of the specs for cache
Fixed up cache.getKeys to strip the prefix and adjusted delete() accordingly

Signed-off-by: Gideon Farrell <me@gideonfarrell.co.uk>