add NestedCatalogNotFound exception
keep track of compressed file handle
Catalog knows about own content hash
extra checks in Catalog._check_validity()
Catalogs can be closed after usage (ulimit -n)
CompressedObject.get_compressed_file()
convenience function cvmfs.open_repository()
get the nested catalog count
traverse catalogs rooted anywhere
python tools can load catalog's predecessor
add python CatalogStatistics::get_all_fields()
Merge pull request #557 from reneme/feature-python_improvements

Feature: Improvements in Python Library

This adds some features I added opportunistically while gathering some statistics figures of various CVMFS repositories.
    Load catalog's predecessor
    Versatile catalog traversal iterator
    cvmfs.open_repository() accepting both local FQRN and remote URL
    Allows for manual catalog closing (saving file descriptors)
    some other minor extensions