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.
Fix warnings from XCode analyzer

Fixes GH-71.
Merge pull request #72 from box/xcode-analyzer-warnings

Fix warnings from XCode analyzer
Kill ODRefreshControl and listen to notifications from the SDK only when appropriate in folder picker
Merge branch 'folder-picker-bugs'

Conflicts:
	BoxSDK.xcodeproj/project.pbxproj
Can no longer build with iOS 5.x SDK

{buildfix}
Update deployment target to 6.0