Fix several folder picker bugs

* Fix a crash when the folder picker is loaded and the SDK becomes
  authorized outside of the folder picker due to view hierarchy
  inconsistencies. Fixes GH-44.
* Reset folder picker navigation title to nil when presenting the
  authorization webview.
* Ignore BoxSDKOAuth2ErrorAccessTokenExpired in error handler. This
  error code indicates the SDK is attempting an automatic refresh.
  It does not indicate a logged out state.
* Fix bug where navigation bar loses styling when the folder picker
  is dismissed and represented.