Use composer to manage bootstrap and jquery dependencies

Composer is a dependency manager: when your project requires a library, it will fetch them and manage their versions and their own dependencies.
It allows you to simply install them and update them using the following commands:

    ./composer.phar install
    ./composer.phar update

The only thing you have to do is create a file describing your dependencies: `composer.json`.
It is a good practice to put the dependencies in a `vendor` directory and ignore it from your repository.

Also, once you have a `composer.json` file, you can register your project on [Packagist](https://packagist.org/), so people will be able to install your project using the following command: `./composer.phar create-project <composer-pacjage-name>`. It will download the project in a new directory, and install the dependencies.
If you wish, I can register your project on Packagist, but you'll have first to merge this Pull Request: Packagist won't accept the project unless you have a `composer.json` file at the root of your project...