[3.4] Composer Integration

Part of the vision for 3.4 is integrating [Composer](http://getcomposer.org) into the CMS.  This pull request as of this time represents a working solution for this.  Changes with this pull request:

- Composer installed code is under `libraries/vendor`
- Third party code which we have manually added that is installable via Composer is handled as such (Symfony YAML component, the password_compat library, PHPMailer, and lessc)
- Framework code we have manually imported is moved into the Composer installation
- The Framework's DI package is updated to 1.2 (we're shipping 1.0 as of 3.3)
- The Platform's `Input` package is replaced with `Joomla\Input` from the Framework with class mapping for B/C (same as what was done for the `Registry` package)
- The build script is modified to only package production files from the Composer installed packages
- `JApplicationBase` now extends `Joomla\Application\AbstractApplication` and now duplicated code or class variables in that class and child objects are cleaned up

TODO:
- Verify the build script is working completely as intended for all package types
- Determine if unused Framework packages (brought in because of dependencies within the Framework code) should be packaged with releases; if so then we should look at perhaps using it and removing the Platform equivalent otherwise we should ensure the build script excludes those files