Manage gems + Ruby at current stable version(s)

The current stable version of Ruby is **[2.0](https://www.ruby-lang.org/en/news/2013/02/24/ruby-2-0-0-p0-is-released/)**. Bitcoin.org's version, 1.9.1, has been [end-of-life'd](http://bugs.ruby-lang.org/projects/ruby/wiki/ReleaseEngineering) and is not supported.

Most popular ruby projects have also adopted [Bundler](http://bundler.io/) to simplify managing their gem dependencies. Bundler lets developers have more than one version of a gem (or of Ruby itself) on their machines without conflict.

This pull request updates bitcoin.org to Ruby 2.0.0 and encapsulates all Ruby dependencies in a `Gemfile` so that one command (`bundle install`) now downloads and install all required gems. Other than making it easier for developers to contribute, there are no changes. Any curmudgeonly developers who do *not* wish to run modern Ruby can simply ignore the Gemfile and everything will work the same (though you really *should* update :).

README has also been updated to reflect the change.