Distribute as a Phar archive

If we were to bundle wp-cli into a Phar archive, instalation would be as simple as:

```bash
curl wp-cli.org/packages/phar/wp-cli-0.9.0.phar > /usr/bin/wp
chmod +x /usr/bin/wp
```

No more PEAR, phing, phix, pyrum and all that noise: #294 

It can't handle bash completions, but it can handle man pages, unlike PEAR: #39