Bump version to 0.22 and update TODO a bit
Cosmetic changes in TODO
Issue #29: Fail more gracefully when no string table is found for dynamic.
BaseException.message is deprecated in Python 2.6 and gone in 3
Use tuples with str.endswith(), supported since Python 2.5
Use integer division in Python 3

This means DwarfConfig.default_address_size is now an integer in Python 3, as
in Python 2.
Implement Section.__hash__() to avoid Python 2’s DeprecationWarning
Avoid BytesWarning in debugging messages in Python 3
Drop unused imports