73 allow user to change language

This PR adds the ability for the user to select the language for the page to be displayed.
The language preference will be set and saved for the user so it's restored every time the user visits the site.
This is how the selection flags are shown in the page:
![home_flags](https://cloud.githubusercontent.com/assets/43977/3086164/78fa0a86-e527-11e3-9b1e-c9e3a7ee6dda.png)
![logged_in_flags](https://cloud.githubusercontent.com/assets/43977/3086165/84f36b0c-e527-11e3-9051-c08dc8d4727d.png)