--quiet-curl flag for rvm to make curl silent using -sS during rvm's fetch operations

This new rvm flag passes the options `-sS` to curl during rvm's fetch operation. From the curl manual:

> -s Silent or quiet mode. Don't show progress meter or error messages.  Makes Curl mute.
> -S When used with -s it makes curl show an error message if it fails.

So --quiet-curl will make curl silent but still return an error status to rvm if something goes wrong.

rvm README and docs/rvm.txt updated to reflect the new option. rvm/rvm-site documentation hasn't been prepared yet.