rename MediaCore CE -> MediaDrop in the documentation
refer to new github repo in the documentation
remove more references to MediaCore / outdated URLs in the docs
MediaDrop rename: update README (and reformat text using MarkDown so hopefully it'll look a bit nicer on github)
update copyright headers to reflect the MediaDrop renaming
rename user-visible strings MediaDrop and use for all links mediadrop.net.

There are still a lot of references to MediaCore (especially internal IDs and Python package names). That change will come later (but not for the 0.10 branch because we won't add such an intrusive change in our stable branch).