joyent/node-manta#119 want "mjob share" subcommand

This commit implements "mjob share", a better version of what we previously called "jobex" internally.  For details, see the man page changes.  Here's an example usage:

    dap@sharptooth node-manta $ cat my-readme.md 
    # Hello, world
    
    Here's an example job.
    dap@sharptooth node-manta $ mjob share -r my-readme.md 5e03c16c-7ccc-4fb4-919e-45fbfe96228a
    https://us-east.manta.joyent.com/dap/public/jobshares/5e03c16c-7ccc-4fb4-919e-45fbfe96228a/index.html

You can view the result at the link above.  The readme is optional.  The HTML file is completely self-contained.  If you want, instead of saving it to your public directory, you can get the HTML directly with:

    dap@sharptooth node-manta $ mjob share -r my-readme.md --stdout 5e03c16c-7ccc-4fb4-919e-45fbfe96228a > out.txt
    dap@sharptooth node-manta $