Merge pull request #67 from intgr/cz

Remove unused gender argument from CZBirthNumberField.clean
Add Latvian (lv) flavor with Latvian municipalities

110 municipalities and 9 republican cities.

Translations will come later.
Add Latvian (lv) postal code field (LV-XXXX)
Add Latvian (lv) personal code field (XXXXXX-XXXXX)