Adding license specification to gemspec

By including the license directive in the gemspec, Rubygems will display
that license on the project's homepage (http://rubygems.org/gems/ezid).

For the license to be displayed, a new version would need to be
submitted (or the previous version yanked and replaced).
Merge pull request #4 from jeremyf/adding-license-to-gemspec

Adding license specification to gemspec