How to add code just below closing HEAD tag of every page?

How can I add a bit of code just below the closing head tag of each page?

This is required to use the new ZenDesk Widget. ZenDesk setup instructs you to:

" Copy the following script and paste it into your website's HTML source code before the closing </head> tag.

This code must be inserted into every page you want to show the widget on. Note that the widget will not slow down your web pages."