Simplify

[anselmdk]

I've found this module useful for quite a few projects already, however it's unnecessary complicated.

Beyond simplifying the code (which I've taken a stab at in 385dfe4), I'd like the interface to dictate less methods.
Right now it dictates the following:

//Dictated by {@see UploadDirRulesInterface}:
function getCalcAssetsFolderDirectory()
{
    return 'apps';
}
function getMessageSaveFirst()
{
    return null;
}
function getMessageUploadDirectory()
{
    return null;
}
function getReadyForFolderCreation()
{
    return true;
}

I think we could get around with only dictating getCalcAssetsFolderDirectory, and leaving the rest as optional. Actually, getReadyForFolderCreation seems a little obsolete, as it can perfectly be handled by getCalcAssetsFolderDirectory returning something, or false.

Not sure if this would be a best practice.