iosapp should prompt for an access token

[1ec5]

iosapp currently requires that you set a MAPBOX_ACCESS_TOKEN the first time you launch after a clean build; after that point, the access token is persisted as a user default. People (myself included) always forget to set this environment variable.

When iosapp is unable to find an access token in user defaults, it should prompt for one. This is what osxapp does, and it's a lot more convenient than what we have now.

If there's a need to automate setting the access token outside of checked-in files, we should add a Run Script build phase that reads the contents of ~/.mapbox and inserts the access token into the Info.plist in the build product (which is not checked in). Alternatively, you could add a command line argument in the scheme editor (right above the UI for setting environment variables) to the effect of -MBXMapboxAccessToken sk.feedcafedeadbeefbadebede. (You can already do this for any user default.)

/cc @kkaefer @friedbunny