Start reachability monitoring when setup
Add support for navigation interstitials

Navigation interstitials are much like URL interstitials, but the URL
represents a screen in the app to which the user can navigate (for
example, to buy a gift card).

Also adds "buttonText" as an action property for both navigation and URL
interstitials. This represents the text that should be displayed inside
the button that opens the URL.
Version bump: 1.6.3

[Enhancements]
* Add support for navigation interstitials

[Bugfixes]
* Start reachability monitoring when setup
Adds merchant factories and stubs to support multi merchant end points
Add support for ordering LevelUp gift cards
Add support for LevelUp iBeacons check-ins
Version bump: 1.6.4

[Enhancements]
* Add support for LevelUp iBeacons check-ins
* Add support for ordering LevelUp gift cards
* Add merchant factories and stubs to support multi-merchant
endpoints