`switch-theme' switches to old theme if argument `new` is nil.

Thus, one can write, e.g. `(change-theme nil 'dark-theme)' to use the
default theme during the day and dark-theme during the night.
`switch-theme' switch to old theme when `new' is nil in pre-Emacs-24.
Merge pull request #4 from gkya/master

Use default Emacs theme if nil is passed as theme.