Added a close button to the Popup class

A formatted button with an "x" that closes the popup when you click on it. Must explicitly set config.closeButton to "true" for it to enable.
Updated buildConfig in PopupManager to copy config

Before, custom configuration options were lost. This change makes custom options pass through. Added to support the closeButton configuration option that I added in my previous commit.
Merge pull request #176 from drezabek94/master

Add a close button to Popup's title bar