Fix "before first heading" error when running outline-cycle

In some files (e.g. outline-magic.el itself), when the point is at the
beginning of the buffer and I run ‘outline-cycle’, the whole buffer gets
hidden. Subsequent runs of ‘outline-cycle’ fail and everything remains
hidden. The failure is caused by (error “before first heading”) called
from `outline-back-to-heading`.

The reason for this error is that in some files or modes, the top-level
outline headings do not have a level of 1 but some higher number. The
`hide-sublevels` command tries to "find" headings of level 1 and it
fails. In this patch, the level of the top-level heading is determined
the same way as in the outline-mode and then `hide-sublevels' is called
with the appropriate level as the parameter.

The error happens and the patch was tested in GNU Emacs 24.3.1
(x86_64-pc-linux-gnu, GTK+ Version 3.4.2) of 2013-04-13 on trouble,
modified by Debian.