Rename "core" to support future devices

[kennethlimcp]

With the photon coming and possible future devices, there should be a new naming convention instead of the current "spark core add" example in the instruction etc.

[KarbonDallas]

this is a great idea, I've got this in my notes. I'll update this issue when we know more about the timeframe of this.

[KarbonDallas]

closing this in favor of #131