Add option for locale [default: get system locale]

Hi,

`locale` is a python system module and thus should work on any system.

This pull request is missing a check over the supported locales, and some sort of fallback system/exception handling in case a locale that is not avaible is requested.

Please find below two example generated with the `en_US` locale:
![wikipedia articles which can be mapped in openstreetmap - mozilla firefox_137](https://cloud.githubusercontent.com/assets/367272/3919094/e1442af4-23a2-11e4-8cc7-2564b08f596d.png)
![wikipedia articles which can be mapped in openstreetmap - mozilla firefox_136](https://cloud.githubusercontent.com/assets/367272/3919095/e145a6e0-23a2-11e4-82af-fa8212389428.png)

Cristian