Reworked memoize

- Removed func.__name__ from key (memoize class is not a singletone thus _cache 
  is always new for next decorated function).
- Removed *args: segments are not receiveing any non-keyword arguments so no 
  need to bother with *args at all.
- Replaced additional_cache_key with cache_key: all functions that formerly used 
  additional_cache_key appear not to need initial arguments as cache keys. It 
  also simplifies the code.

Ref #205
Fixes #208