README: Rewrite in Markdown
README.md: change Subversion repos to git repos

Both my own and the libEBML/libMatroska repositories have moved to git a
long time ago.
debian-upstream: README is README.md now
fedora-core: README is README.md now
centos: README -> README.md
opensuse-12: README -> README.md
Update mkvmerge-gui.html
Update mkvmerge-gui.html
Update mkvmerge-gui.html
Update mkvmerge-gui.html
INSTALL: update name of README.md file
Build system: rake target for building a source tarball