Add script to generate release checklist

[jhnstn]

Adds a script to generate the release checklist. Once the checklist is merged, the script will fetch the checklist from trunk by default.

Currently the version, host app version and release date are parametrized. Try ./release_checklist.rb -h for the help menu.

Testing:

1. run ./release_checklist.rb 1.2.3 -t ./templates/release_checklist.html.erb
2. Try posting the result to a p2
3. Verify that all the blocks look correct