Database Names

Currently it is a bit akward to provide the database name. It works like this:

```ruby
database = Ashikawa::Core::Database.new do |config|
  config.url = "http://localhost:8529/_db/my_database_name"
end
```

I think this should be an option, but I think we should offer another way as well:

```ruby
database = Ashikawa::Core::Database.new do |config|
  config.url = "http://localhost:8529"
  config.database_name = "my_database_name"
end
```

While we're at it, we can adjust the way we handle the `_system` database in ArangoDB: When no database is provided, the API String will be extended to be "http://localhost:8529/_db/_system". This can lead to some nice refactorings.