Change URI for XML namespace

[donmendelson]

Use the fixprotocol.io domain as the root of XML namespaces. That domain is specific to FIXTradingCommunity organization in GitHub. The suggested format is to append version, for example 'http://fixprotocol.io/sbe/rc3' for Release Candidate 3.

As currently stated in the spec, namespace URI is only for unique identification, not intended as a URL to be accessed at application run-time. However, convenient links will be provided for developers to download the XSD file at development time.