Refactor the gem spec out into a proper .gemspec file per ruby gem convention

Also pulling into bundler's gem tasks to provide the `build` task

The .gemspec has the added benefit of making it such that one can use bundler
to reference this gem by :path or :git in the Gemfile, e.g.

    gem 'facter', :git => 'git://github.com/puppetlabs/facter.git'
    ...

This will make it easier to work with non-yet-released versions of facter