Handle resource updates & use modx cache folder

[christianseel]

I moved the statcache folder to modx core/cache folder (I think this makes sense).

I also implemented some basic functionality to handle updates of a resource (delete the static file in the cache). This code might be improved before you build a new package version.

[sepiariver]

@christianseel does this mean you have to provide access to the core folder? what if core/ is above the web root? Just curious....

[christianseel]

@sepiariver you're right, it won't work if the core folder is above the root. So it makes no sense to move the folder into core/cache/ :)

[christianseel]

Closing this because #11 is better ;)