centrally include Manifest classes
FIX: Exceptions didn't inherit from Exception
TEST: unit test for cvmfs.Manifest
split_md5() function in _common.py
add catalog retrieval methods to Repository
add convenience imports to __init__
add _common._combine_md5
cache catalogs after first open
add Catalog and DirectoryEntry
TEST: unit tests for md5 splitting combination
allow for interactive inspection of catalogs
FIX: Catalog._canonicalize_path() screwed up empty path
add additional dirent flag checks
make a whole repository iteratable