Update gem, mainly add tests

[Koronen]

Hi Peter!

I've made some updates to your gem:

• Added bundler to handle development dependencies
• Added rspec and wrote some tests for public methods
• Added guard for automatic testing
• Added cover_me to show test coverage

I've also made slight modifications to a couple of methods that made sense to me:

• initialize now throws ArgumentError when an invalid argument is passed
• age is now (re)calculated on request, since it is time dependent
• to_s always returns the standard form, e.g. 'yymmdd-xxxx'
• Fixed a suspected typo for the "Värmlands län" region

Lastly, I also bumped the version number, since version 0.0.4 is on rubygems already.

I'll gladly accept feedback.

Cheers,
Victor Koronen

[peterhellberg]

Really nice work! (Holman helped me figure out why I didn’t get any notification of your pull request)