Add parent env functionality

I have add the possibility to specify a parent environment.

```json
{
    "Base": {
        "AppID": "123",
        "Host": "https://foo.bar"
    },
    "Production": {
        "Host": "https://more.foo.bar",
        "Logging": 0
    },
    "Sandbox": {
        "Logging": 1
    }
}

```

```objc

[DBEnvironmentConfiguration setEnvironment:@"Production" parentEnvironment:@"Base"];
[DBEnvironmentConfiguration valueForKey:@"AppID"]; // @"123"
[DBEnvironmentConfiguration valueForKey:@"Host"]; // @"https://more.foo.bar"
[DBEnvironmentConfiguration valueForKey:@"Logging"]; // 0
```

```objc
[DBEnvironmentConfiguration setEnvironment:@"Sandbox" parentEnvironment:@"Base"];
[DBEnvironmentConfiguration valueForKey:@"AppID"]; // @"123"
[DBEnvironmentConfiguration valueForKey:@"Host"]; // @"https://foo.bar"
[DBEnvironmentConfiguration valueForKey:@"Logging"]; // 1
```