Rainfall update

**Add new triggers:**
+ average rainfall
+ average annual rainfall

**Add support for [Alpha-3 country codes](https://en.wikipedia.org/wiki/ISO_3166-1_alpha-3):**

![selection_027](https://cloud.githubusercontent.com/assets/5282264/4954774/b73445de-668f-11e4-96e3-086b7cb7a63c.png)
This line is ugly :unamused: but is required as the `Locale::Country::rename_country` method only works for `code2country LOCALE_CODE_ALPHA_2`