Add .ruby-version, .ruby-gemset

The `.ruby-version` is a good hint for any user or fellow developer.

Besides, I recommend you to use RVM or rbenv to keep your system gemset clean.
See http://rvm.io and https://github.com/sstephenson/rbenv
Add Gemfile (and RSpec, but that's not the point)

Providing a `Gemfile` is a simple yet eficient way to:

- keep track of the app dependencies
- lock their versions!
  (Who knows if our tests will run with RSpec 5.x.x in 8 months?)

IMHO: Any modification of the `Gemfile` MUST be committed with
the resulting `Gemfile.lock`.
Running `bundle install` is enough to generate an up-to-date `Gemfile.lock`.

One exception to that rule: `bundle upgrade <package>` which updates the gems
versions without modification of the `Gemfile`.
Add the dependencies documentation

They're all in the Gemfile!
Add lock on the Ruby version

In fact setting the Ruby version in the Gemfile is considered quite
a good practice. It ensures the same version is used in every distinct
environment (development, staging, production), and is aligned with
the 12 factors methodology.

See https://12factor.net
Add Thin

See http://www.sinatrarb.com/intro.html
Fix: remove sensitive data from version control

Be cautious, if you add a file to the `.gitignore` AFTER you
checked it into version control, the file doesn't get removed until
you do it manually, and its changes keep beign tracked :(

To provide a config file example, the best way I know is to use
a distinct file. While the original is ignored, that example file stays
under version control and ensures that users:

- know they should create the corresponding config file
- are aware of defauts (when possible),
  or at least are aware of the file format
Add README

This is the last step, but should be the first. It's one of the most
important files when aiming at peer collaboration. Writing a README is
an unique occasion to show you fellow developers that your care ;)

Ready for some README-driven development?
See http://tom.preston-werner.com/2010/08/23/readme-driven-development.html

Fixes #1
Merge branch 'add-collaboration-helpers'