[ANNOUNCE] Source migration to GitHub

As of now the ical4j source code resides in GitHub repositories here:

https://github.com/ical4j/

The reason for migrating the code is the improved support for community contributions via forks and pull requests. We will probably make use of GitHub's issue tracking also, however we will still try to support any patches/bugs submitted in SourceForge. Also note that SourceForge will still be used for release binaries.