Update and document i18n-tasks 

Update [i18n-tasks](https://github.com/glebm/i18n-tasks) to v0.3.6 and document how to get missing keys with it

* Remove `load "tasks/i18n-tasks.rake'` from Rakefile since i18n-tasks is now an executable.
* i18n-tasks.yml.erb -> i18n-tasks.yml. Available locales are now picked up automatically