NEW errors handled in a specific manner

* Configurable is_error_callback
* configurable error_validity
* errors never get to stale state
FIX minor fixes
OPT Do not refresh valid values with errors

* Entries whose refresh produces an error are let expire without refreshing anymore
NEW added erl_cache:list_cache_servers/0
CHANGE s/stale/overdue/g
DOC Added a reference to memoization
OPT minor updates
OPT Added apache 2.0 license
DOC Update README
Merge pull request #10 from spilgames/readme-update

DOC Update README
DOC added a note about memory management in the README