Made common structure cache.

[TriAnMan]

This cache speedups menu generation on non yet cached pages. Cache time recived from the system's 'cache_expires' setting.

Signed-off-by: Anton Andersen [email protected]

[jpdevries]

@opengeek is there anyway you could help me review this one? I've got a testing branch at https://github.com/modxcms/Wayfinder/commit/7c3a1ad6d2e870760323c5dcf65062a98e147892