Make UIApplicationExitsOnSuspend configurable. Not doing anything to restore the value that was previously in the file, because we use defaults to save the value and therefore don't care about the previous value.

Set this by adding a key under "iphone" in build.yml named UIApplicationExitsOnSuspend. Possible values are not specified at all, true, false, 1, and 0. All others will raise an error and stop the build process.
According to the iOS documentation, the default is false.
https://developer.apple.com/library/ios/documentation/General/Reference/InfoPlistKeyReference/Articles/iPhoneOSKeys.html#//apple_ref/doc/uid/TP40009252-SW23
Merge pull request #263 from ziggythehamster/rhodes-4-ios-uiapplicationexitsonsuspend

[iOS] Make UIApplicationExitsOnSuspend configurable.