Use `rhc cartridge add`

`rhc app cartridge add` is deprecated.
Merge pull request #14 from jroes/patch-1

Use `rhc cartridge add`
Update 'rhc cartridge' command usage
Merge branch 'master' of ssh://test-walden.rhcloud.com/~/git/test

Conflicts:
	README.md
Making twitter service name lowercase.
Adding full text of Moby Dick for upcoming WebSockets example.
Adding websockets example.