Make relative symlinks. Add support for subdirectories.

Relative links are more portable (imagine chroots, for example). The
a2ensite script uses relative links, too. There should not be any
disadvantages in comparison to absolute links.

In some cases, you might want to organize your sites in subdirectories.
This patch provides support for this.
man page: Fix default example.
Merge pull request #3 from maix/master

Relative links, subdir support