Added geolocalized tweet button

I've added a handmade tweet button to tweet my snow, I hope you'll like it.

![](http://cl.ly/image/0f0L0i1o1T1M/Capture%20d%E2%80%99%C3%A9cran%202013-02-10%20%C3%A0%2023.21.39.png)

![](http://cl.ly/image/3E0a2c3Q1L0p/Capture%20d%E2%80%99%C3%A9cran%202013-02-10%20%C3%A0%2023.28.29.png)

**Notes:**

- I'm using the [Nominatim Reverse Geocoding API](https://wiki.openstreetmap.org/wiki/Nominatim#Reverse_Geocoding_.2F_Address_lookup), I think [they're cool](https://wiki.openstreetmap.org/wiki/Nominatim_usage_policy) but I'm not 100% sure
- It's using the native browser capability for detecting user's coordinates, so it will ask for authorization (a fallback is provided if browser is <del>old</del> IE)
- Geolocation accuracy strangely vary between browsers
- I didn't manage to put the twitter tweet page into a bootstrap modal, mostly because of CORS restrictions and hair-pulling
- Wording is up to you
- It works from a mobile device (iPhone 4, iOS 6.1)
- I tried to write this PR in French but it felt too weird, I'm sorry.