Use Composer Dependency Manager

The Cloudant, SendGrid, and Twilio examples should use [Composer](https://getcomposer.org/) for dependency management, rather than having their dependencies committed to the repository. This pull request removes the respective libraries, adds a `composer.json` file to each example which lists each project's dependencies, modifies each example to use Composer's autoloading, adds a README with instructions to build using Composer, and ignores the `vendor` directory in both Git and Cloud Foundry (the build will happen on deployment).

I successfully tested deploying the Cloudant example to Bluemix after making these changes. However, I did have to change `myCloudant` to `Cloudant NoSQL DB-ok` in my `manifest.yml` file (this change is not part of this pull request, as it is out of scope).

The vendor directory should really be moved out of the document root as well. The simplest way to do this would be (for each project) to create a new document root (e.g. `public`) and place only the `index.php` file in this directory. Let me know if you'd like me to send a pull request for this change as well.