Create binding to download Foreman resources

## Need

```gherkin
As a developer
I want to download a Foreman instance resources as JSON objects
So that I can  describe the instance as a Git repository
```

## Deliverables

- A binding to make API calls to the Foreman instance and download the resources as JSON objects.
The objects will be dumped as files in the Git repository via the ConfigurationManager #6 
- The binding will be used by the synchronization cron (ran by the Foregit daemon) or via a command from CLI.
- The repository structure should be configured via a setting, to describe which resource should be downloaded in which directory and the file name formats.

## Solution

#### Prerequisites
- apipie-bindings - use it to make API calls to download the resources


#### TODO
- [ ] create Foreman::Download class to make API calls to download the resources:
  - create a list of resources which will be downloaded: architesures, hosts_groups, OSs
  - for each resource make API calls to download them, and call the ConfigManager class to save them as files 

#### Files
- [ ] lib/foreman/download.rb

@iNecas I would like to mock API results to write tests, but I couldn't find an easy way to use FactoryGirl for this.
 Do you know what should I do? 

In factory-boy is so easy...