Recommend license attribute in gemspec

Per discussion in https://github.com/rubygems/guides/pull/67

1. Adds a license attribute to the example gemspec
2. Adds categories for Required and Optional gemspec attributes sections
3. Add 'Recommended' category for license/licenses within the Optional section
4. Add documentation to license, re: github chooser and legal importance
5. Add recommendation to warning (and test)

@drbrain I built the spec guide and it looked fine. `RUBYGEMS_DIR=$HOME/repos/forked/rubygems rake spec_guide --trace`