Adds a copy of the GPL Version 3.
adapt copyright rake task to include all own files

Now includes:
 * .rb
 * .rake
 * .feature
 * .html.erb
 * .js(.erb)
 * .css(.erb)
 * .api.rsb

 It should exclude files and directories that we include but for which
 we do not hold the license.
provide every possible file with a short copyright notice

This was done using the rake task: rake copyright:update
replaces bundled prototype_legacy_helper by gemfile reference