Add docstring for LoadContainer

Closes #31
Docstring for ImageDisplayStuff

Closes #32
Docstring for ImageTreeWIdget

Closes #33
Fix docstring typo
Add docstring for FItsViewerWidget

Closes #35
Document ImageBrowserWidget

Closes #37
Document ToggleContainerWidget properties

Closes #38
Docstring for ToggleGoWidget

Closes #39
Document ToggleMinMaxWidget

Closes #41