Make phar installer througn bash script

API to get latest release assets:
https://api.github.com/repos/sokil/php-mongo-migrator/releases/latest

1) Add bash installer
2) Update manual:

#### Install phar

Run in shell:
```
wget http://phpmongokit.github.io/dists/mongo-migrator.phar && chmod +x mongo-migrator.phar && sudo mv mongo-migrator.phar /usr/local/bin
```

