Add ios image commands to REST API

[danielpaulus]

We need the capability to list, mount and automatically set up developer images using the REST - API.

I propose to use the device middleware and the LimitNumClientsUDID middleware and set up the following endpoints with tests and good swagger documentation: GET /device/:udid/image PUT /device/:udid/image PUT /device/:udid/image?auto With clear and detailed error messages for all possible failure cases. For writing tests it is totally ok to mock out go-ios. Just testing that the endpoints call the right functions in ios.imagemounter is enough.

The code can go to device_endpoints.go I think, but I am not too opinionated here.