Make loadSchema(io.Reader....) public

[onitake]

In some situations, it is desirable to allow loading a schema from something else than the supported URIs.

For example, a program could have XSD definitions built in, so they don't have to be fetched from a file or the internet.

Please make loadSchema(r io.Reader, loadUri, localPath string) in xsd-schema.go public or add a new constructor that allows passing in a map of readers for the included schemas.