Combining the above, do you need someone to build it for you, or are you willing to do things yourself (with help when possible)?
That's rather vague, care to elaborate? What kind of 'app' where you thinking about? Does it have to be a native app, and if so, what operation system? Could it also be a web app, and if so, what are the possibilities there (LAMP stack, node.js)? Does the data need to be stored somewhere, as to make a hystorical overview, or do you just want the current readings, starting from when you started the app.
Is this just a boolean ON/OFF state, or are you expecting something else? What are you planning on measuring with the Core?
There are also some very nice only services out there which you can use to store/visualize data. There's a topic for using google here:
Ubidots is also said to be rather nice. Atomiot is created by one of the community members, and also includes data logging/visualisation.