Update and document i18n-tasks

Update i18n-tasks to v0.3.6 and document how to get missing keys with it

* Remove load tasks/i18n-tasks.rake from Rakefile, i18n-tasks is a
  binary now.
* i18n-tasks.yml.erb -> i18n-tasks.yml. Available locales are now picked up automatically
Merge pull request #421 from glebm/i18n-tasks-doc

Update and document i18n-tasks