Fix for gettext rake tasks

These commits are fixing issues with gettext rake tasks, especially
* rake gettext:find
* rake gettext:add_language[XX]
*(where XX is the ISO 639-1 2-letter code for the language)*

To run those rake tasks, you need to define in your Gemfile.dev.rb:
```ruby
  gem "gettext",  "~>2.0",  :require => false
  gem "locale",   "=2.0.9"
  gem 'ruby_parser',        :require => false
```
Still not working
* rake gettext:store_model_attributes

The paths in $LOAD_PATH, and also Rails.configuration.paths or autoload_paths are defined as strings, not instances of the Pathname. Anyway it's possible that the error appears in context of gettext rubygems.