Bring back Phar archives

@DavertMik made a [convincing case][1] for providing a Phar archive as an installation option.

In summary:

If you're using WP-CLI yourself, you should install it via Composer.

If you want to use WP-CLI as a tool for automation, like say in a Travis CI script, it makes more sense to download a single Phar archive. (We could even use this to replace https://github.com/wp-cli/wp-cli/blob/master/templates/install-wp-tests.sh)

A `wp self-update` command (#522) would need to take into account if it's installed as a Phar archive or not.

[1]: https://github.com/wp-cli/wp-cli/pull/439#issuecomment-22315750

The plan is to have two Phar files:

* wp-cli.org/builds/phar/wp-cli-stable.phar
* wp-cli.org/builds/phar/wp-cli-nightly.phar

Tasks:

* [ ] revert 8483fb709b2285a033515f59b4ad818468a0372a
* [ ] see if using Symfony\Finder makes sense
* [ ] see if we can build the Phar file after each commit via Travis:
    * http://about.travis-ci.org/docs/user/deployment/
    * http://about.travis-ci.org/docs/user/encryption-keys/