Differentiates DEBUG and TestFlight builds. Resolves #26

[jazzychad]

• If the current build is a debug build, SignalPayload.isTestFlight will now return false.
• Adds an isDebug payload key

[jazzychad]

⚠️ A couple of potential issues with the way this is implemented:

• It is possible (but probably rare) that a developer would make a testflight build in a Debug configuration (by changing the Archive setting in the scheme from Release to Debug). This would defeat the debug/testflight detection implemented in this patch.

• I'm not sure how necessary it is to add an isDebug key to the payload b/c presumably it will only be relevant for a very small number of signals actually sent to the server (and can be inferred from other payload key values). I am happy to remove that isDebug key addition if would make more sense.

[winsmith]

Thanks a lot for this contribution <3