I’m using GIMP 2.8.1. I hope this will work for you:
Open the “Windows” menu and select “Single-Window Mode”.
Simple 😉
In the detached window (Tool Options), the name of the view (Paintbrush) is a grab-bar.
Put your cursor over the grab-bar, click and drag it to the dock area in the main window in order to reattach it to the main window.