Update cocoapod related files

No new pod is added to the Podfile, but I ran
`pod install` to make sure that required files
aren't missing since I was running into issues
while trying to run the tests.
Start tracking Branded.xccheckout file

Not sure why this file wasn't created / tracked when
the project was created originally. I tried to remove this
file, but Xcode keeps re-generating it. Since this file
is in xcsharedata directory and none of my personal Xcode
related user data is included in this file there is no harm
in tracking it.
Add missing test images

I forgot to add these images to the project. As a result,
the tests for MDMenuItem class were failing.
Ensure sharedInstance returns the same instance