First go at a DB adapter for MongoDB

Execute the following to get a mongo server working inside vvv for testing:
```
vasshin # ssh into vagrant
sudo apt-get install mongodb -y
sudo pecl install mongo
echo 'extension=mongo.so' | sudo tee -a /etc/php5/mods-available/mongo.conf
sudo php5enmod mongo
sudo service php-fpm restart
```

You'll need to install a mu-plugin so the db engine would switch, put the
following in `mu-plugins/stream-mongo.php` ( or any filename ):
```
<?php
add_filter( 'wp_stream_db_adapter', function() { return 'mongo'; } );
add_filter( 'wp_stream_no_tables', '__return_true' );
```