Add web_console gemspec

Back in the days I haven't read the [name your gem] guide. We have a
chance of fixing the name before release.

[name your gem]: http://guides.rubygems.org/name-your-gem/
Idea from https://github.com/codahale/bcrypt-ruby/pull/86

```

  #######################################################


  The `web-console` gem has been renamed to `web_console`. Instead

  of installing `web-console`, you should install `web_console`.  Please

  update your Gemfile and other dependencies accordingly as the legacy

  `web-console` gem will not be updated after version 3.0.


  #######################################################

```