Update to be able to use the newer zip-container gem.

Can use either version 0.9.0 or 1.0.0 now so we can stay compatible a bit longer
with other code that uses the old rubyzip API.
Add a version badge to the ReadMe.
Add ability to use a custom mimetype.

This is for subclasses of UCF-type documents.
Verify META-INF/container.xml against its schema.
Verify META-INF/manifest.xml against its schema.
Add META-INF content to the example ucf file.

Added container.xml and manifest.xml files and made sure the resulting UCF file
validates.