Create gem building mechanism

Hi, this looks really useful, I find hiera-gpg a bit too heavyweight and like your approach.

I've added a simple gem builder for you - (edit the email address in Rakefile if you want to)..

$ rake gem  - to create a new gem at any time (update the version number in Rakefile first).

Please publish to rubygems.org when you're happy with the first one.

I've also added a more useful error message when it all goes wrong, and made a keys directory to store public_keys so it can be ignored by .gitignore. See what you think.

Tested this all out first, makes me life a whole lot easier!