Add lock endpoints to reserve devices (#187)
This is the first batch of code for #182
Added POST /reserve/:udid endpoint to reserve a device by provided UDID
Added DELETE /reserve/:udid endpoint to release a reserved device by provided UDID
Added GET /reserved-devices endpoint to get a list of the currently reserved devices
Added a goroutine that will check reserved devices every minute and remove devices that were not used for more than 5 minutes
Added API tests for the new package, not really sure if that's what was meant by 'integration tests'
