Add installer script

The idea is to simplify the Phar archive install process to a single line:

```
curl -sS https://wp-cli.org/installer | bash
```

It would ask a single question (and default to whatever `find-php` returns):

```
Found PHP path: /usr/bin/php/
Is this correct? [y/n]: 
Path to PHP binary: 
```

And then do:

```bash
curl http://wp-cli.org/packages/phar/wp-cli.phar > /usr/share/wp-cli.phar
echo "$php_path /usr/share/wp-cli.phar \$@" > /usr/bin/wp
chmod +x /usr/bin/wp
```