Tweaks based on comments by @nviennot

types at the command line are lowercase
':object' is now the dummy type
Generated models now include NoBrainer::Document::Timestamps by default
index declarations are inline with fields
The generator wasn't accepted, so the app creation section is more explicit
Rewrote an awkward sentence
Minor tweaks to rails tutorial
Merge pull request #427 from rethinkdb/josh/nobrainer_updates

Tweaks to rails tutorial based on comments by @nviennot