Replaced the custom, untested memoize with a similar decorator from Python's
3.2 stdlib. Although some minor performance degradation (see ticket), it is
expected that in the long run lru_cache will outperform memoize once it is
implemented in C.
Thanks to EvilDMP for the report and Baptiste Mispelon for the idea of
replacing memoize with lru_cache.
Previously, when collecting static files, the files would receive permission
from FILE_UPLOAD_PERMISSIONS. Now, there's an option to give different
permission from uploaded files permission by subclassing any of the static
files storage classes and setting the file_permissions_mode parameter.
Thanks dblack at atlassian.com for the suggestion.
