Lazily creating the FLEXWindow when it's actually needed.

This allows users of FLEXManager to use the -registerGlobalEntryWithName:objectFutureBlock: API in a +load method of their app. Right now that creates the UIWindow object when UIKit is not ready. With this change, the window will be created the first time you call -showExplorer, which would also help a bit with launch times in apps that have FLEX because creating that object would be delayed from app launch to the first time it's used.
Also create the explorer view controller lazily.
Merge pull request #14 from JaviSoto/feature/lazy-window-creation

Lazily creating the FLEXWindow when it's actually needed.