Apps can control how they exit

Companion to: Gracefully exit the app

https://github.com/calabash/calabash-ios-server/pull/69
Make spec/Resources a singleton
Adds current_xcode_version to spec/resources.rb

Needed for regression testing.
Use spec/resources.rb to detect alt Xcode versions
Updates calabash_exit documentation
Adds 'opts' argument to calabash_exit
Spec for calabash_exit on simulator
Regr. tests calabash_exit targeting sims
Need an .ipa for on-device testing
Adds ideviceinstaller support for specs
Adds calabash_exit on-device specs
Merge pull request #520 from calabash/feature/calabash-exit-with-options

Apps can control how they exit