use bundler's rake tasks for releasing
Merge remote-tracking branch 'origin/master' into bundler-release

* origin/master:
  States that .wiki files are rendered using Mediawiki syntax
  Release 1.2.1
  Release 1.2.1
  Fix docs for testing in GitHub
  bin is not ignored
  installation and usage before sanitization
  Move link references into contributing
  Add docs for releasing a new version
  Move contributing docs to CONTRIBUTING.md

Conflicts:
	github-markup.gemspec
Merge pull request #291 from github/bundler-release

use bundler's rake tasks for releasing