Show video language, and allow video language selection

Fixes #859.

Two parts:

1. Create a generic i18n template filter for getting a language name from language code, and use that wherever we need to do that.
2. For `video.html`, if there is more than one language available, 

    * Show the video language
    * If the language is clicked, show a dropdown to select a different language.

4 languages available: 
![image](https://f.cloud.github.com/assets/4072455/2483648/522894fc-b103-11e3-85b4-1c7796391d54.png)

After clicking:
![image](https://f.cloud.github.com/assets/4072455/2483653/5efa8cf8-b103-11e3-965b-6ce2cef93321.png)

After selecting `Polski`:
![image](https://f.cloud.github.com/assets/4072455/2483681/bf65247c-b103-11e3-95df-cd17e8d3569d.png)

When 1 language available, language doesn't display, and isn't selectable:
![image](https://f.cloud.github.com/assets/4072455/2483660/6ba8505c-b103-11e3-8dfd-ba6cb374b736.png)

**Testing:**
* Tested when 4 languages available and 1 language available.

**Note:**
* This fix doesn't work when video_language = "pt"; need to think more about best way to handle that.