Better language selection dropdown

This adds a dropdown with an integrated search field, making it easier to select a language. According to the documentation, it should be disabled on touchscreens beacuse it’s not usable, but someone should check it just to be sure.
![tatoeba-chosen-dropdowns](https://cloud.githubusercontent.com/assets/5107734/3628510/1a1079dc-0e95-11e4-8bd6-42140ebaf7b6.png)

While it’s basically working, there are still some little problems. There are still differences between the regular dropdowns and Chosen’s. Notably:
* The width of Chosen’s is reduced to the width of the closed dropdown, making many language names to take more than one line, whereas regular’s extend to the needed space.
* Page up/down keys aren’t working with Chosen’s.
* I was unable to make the search button vertically centered regarding the dropdown, when they are horizontally aligned like in the search bar. The button is a little bit too high.
* Using IME tends to brings problems that I [partially solved](https://github.com/jiru/chosen/commit/cfcdc325f5cb5e474cca318d5e21094797e380df) (I sent a pull request to upstream). On Firefox, when you’re on the keywords field and you jump into the dropdown using the tab key, typing something using an IME doesn’t opens the box and nothing gets selected. I except we will hit other problems related to this.
* The displayed text when no languages are found isn’t easily localizable because it consists of `<localized text> "<keywords>"`.

Also, please confirm me I’m not doing anything wrong in cc8ca2fa96b1b3a9a4a7a2e04976e5d91eee5d98.