Make AssetAdmin obey file permissions

This updates Silverstripe 2.4.x's AssetAdmin to obey per-file/folder permissions. 

NOTE: I noticed that Silverstripe 3's AssetAdmin checks a canAddChildren() function. This patch doesn't use that, as I'm assuming that this is new in SS 3. If this is not the case, then this patch will have to be updated.
Merge pull request #952 from hdrlab/patch-1

Make AssetAdmin obey file permissions