Add Rails compability

At first we want to make sure a `Railtie` exists and we provide access to the configuration in the Rails configuration context. Additionally we provide a way to autoload a YAML file with ArangoDB connection parameters which should be environment dependent. Guacamole should use the `Rails.logger` if not otherwise specified.

**TODO:**

 - [x] Implement `Railtie`
 - [x] Hook into Rails configuration context
 - [x] Load `guacamole.yml`
 - [x] Make configuration environment dependent
 - [x] Use `Rails.logger` per default