Pod cleanup, KVO crash fix, progress bar fix

[alexhillc]

• Cleanup Pod files by adding the Pods directory to .gitignore (a 'pod install' will be needed after cloning, but this reduces clutter in the repo generated by the Sample app)
• Retain reference to navigation bar to fix temperamental KVO crash on dealloc.
• Clear progress view in viewDidDisappear:, dealloc as well as viewWillDisappear: - just in case there is a situation in which we receive delegate progress callbacks.
• Add parens to statement causing warning in Xcode 8.3

[sgonzalesallan]

I think this pull request will address the issue in progress bar bug when pushing the web view controller, as well as the bug in the activity indicator when the controller is dismissed while loading. Good job @alexhillc 👍