Breaking changes

The shared assets have been removed to a private repo. Also, this updates the required platform to iOS 11.

Additions & updates

Alerter protocol with SwiftMessages adapter
Debugging protocol, with some refactoring of existing debugging flow/view controllers
random number extensions on Int/Double
add disclosure indicators on some custom UITableViewCells
typaliases for some common closure types
extension on UIDevice for determining if running in simulator
more use of the Environment struct internally, and renamed some of its properties
updates the Travis.ci image to use Xcode 9.2
updates to InAppPurchaseVendor protocol