Catch and log unhandled exceptions in async tests
Remove test.odt and ignore it in git

This file is always created prior to use in tests.
Add clarification comment on how the meta element behaves (and test)
Don't stop parsing an ODF unless the container state becomes invalid

An error is not necessarily fatal unless the handler sets the state to
invalid as a result of the missing data. In addition, when the state
is set to invalid, log the component that failed to load and (if
defined) the error that has occurred.
Handle documents with no office:settings element

Settings.xml and office:settings are both optional according to the
ODF specifications. WebODF must be able to handle loading and saving
files with or without this setting.
Ensure all ODF elements are recreated when setting a new root element