Overlay a button over the post tab to achive the appearance we want.

Uses placeholder images for highlight stats. We need to update these.
Keeps the graphic on the post tab so the UI is consistent when the button is hidden during rotation.
Merge branch 'develop' of github.com:wordpress-mobile/WordPress-iOS into issues/845-custom-post-tab-button
Fix typo. Use CGRect methods.
Commenting some code.
Merge pull request #846 from wordpress-mobile/issues/845-custom-post-tab-button

Issues/845 custom post tab button