factor out SimpleCatalogManager from WritableCatalogManager
add scaffold for swissknife command 'dirtab'
dirtab processing in cvmfs_swissknife
print warning for useless dirtab rules
FIX: skewed access rights of .cvmfscatalog
FIX: recreate manually deleted .cvmfscatalog
replace dirtab processing in cvmfs_server
FIX: dirtab processing needs scratch dir
TEST: adapt integration test case 544