Add a rake task for generating the gemspec files

`rake gemspec` now generates `kramdown.gemspec` locally for use.

This will help people using a local kramdown reference, i.e.

`gem 'kramdown', :path => '../kramdown'`

As no generic .gemspec is maintained within the repository, it will *not* help people wanting a specific commit, pull request, or the latest dev version, such as:

`gem 'kramdown', :git => 'git://github.com/gettalong/kramdown.git'`

We would love to see a .gemspec in master, as it would allow much more flexible versioning and pull request testing for users of bundler, but it's out of scope for this request.