Added update tool for cjdns

Added ./update, which is capable of merging changes from cjdelisle/cjdns if git is installed, or downloads the tar.gz archive with curl or wget. If it must use curl/wget, it then opens the archive, moves the files into place, replacing the old ones (it does this by writing the move commands into a new shell script that is not part of the update, and then this file is later deleted, see lines 2-14, and then the same method is used in lines 16-21). Finally, it executes ./clean and the ./do to rebuild from source.