Simplify calculation of device type

The problem is that using current code it is not possible to configure Appium as a Selenium node using this configuration:

```js
{
  "capabilities":
      [
        {
          "browserName": "iOS",
          "version":"7.0",
          "maxInstances": 1,
          "platform":"MAC"
        }
      ],
  "configuration": {}
}
```

And then launch Safari browser using this capabilities (and there is no another way to do it either).

```js
{
  browserName: 'iOS',
  version: '7.1',
  platform: 'MAC',
  platformName: 'iOS',
  platformVersion: '7.1',
  deviceName: 'iPhone Simulator',
  app: 'Safari'
}
```