Update curl flag and add additional two-step installation instructions.

Updating curl flag and instructions to follow better security practices
used by other projects: https://github.com/saltstack/salt-bootstrap

Other references:
http://www.reddit.com/r/linux/comments/1s58my/please_stop_piping_curl1_to_sh1/
http://www.seancassidy.me/dont-pipe-to-your-shell.html
https://news.ycombinator.com/item?id=8550511
http://output.chrissnell.com/post/69023793377/stop-piping-curl-1-to-sh-1
http://www.reddit.com/comments/1pqtcb
Should just use `-O` instead of `-o rustup.sh`