Try to fix relative path issue at file inclusion

But I think I gonna abort that method soon, it seems not to work…
Get Absolute path everywhere

Absolute paths are used for every include in inc/ files because they could be included by files from different directories.
When not in Freeder root directory, a path.php file MUST define ROOT_DIR correctly if some files from it can be called by the user. (see api/)
`constants.php` is now in root because:
 * It is more easy to find for newcomers (and it is a useful file for little hacks)
 * It defines where to find `inc/` so it cannot be in it!
Default theme is now defined in `constant.php`
Merge branch 'master' of https://github.com/FreederTeam/Freeder