Add support for multiple reserved names and fix checks.

Can now have (and test for) more reserved names in the UCF document. The
original method to test for clashes with the reserved names was also not working
so this has been fixed and tests added.
Ensure UCF document is closed after verifying it.
Expose the "each" method for enumerating UCF entries.

This is just the underlying Zip::ZipFile#each method forwarded through.
Expose the "size" method to count UCF entries.
Add an example program to list UCF contents.

ucfinfo is similar to a much cut-down zipinfo. It simply lists some information
about the UCF file and lists the files within it.
Add a method to directly iterate over UCF entries.

The UCF::Container%each_entry class method is equivalent to the
Zip::ZipFile%foreach class method.
Update changes file and gemspec for release.