Use node-plist for plists; settle platform-vs-brand identifier distinctions.

We don't have to use special qualifiers on the target plist element,
then.

Identifiers issue: different platforms of the same brand could use the
same identifier. We have inherited distinct and peculiar identifiers for
android ("com.spideroak.android") and iOS ("com.spideroak.SpiderOak"),
which means we will have to maintain distinctions for these
platforms.

 However, we can avoid having to change existing project_configs when
 new platforms come around by settling on a simple form for a default
 identifier, and sticking with that. What we've resolved:
- android and ios will have their distinctive identiers
- the default identifier will be "com.spideroak.mobile"