Add custom target in order to create proper tarball

[Olivier Duchateau]

Patch create easily tarball for next releases.

Currently when I extract tarball downloaded from launchpad or midori-browser sites, contents is extracted in current folder. The custom target creates proper tarball with following schema 'midori-version'

[Cody Garver (codygarver)]

I recommend TXZ compression instead (.tar.xz). But it only works with CMake >= 3.1.0.

[Olivier Duchateau (duchateau-olivier)]

I know TXZ is better, but I followed file compression format currently used.

[Cris Dywan (kalikiana)]

A merge request would go a long way to merge this. :-)