Implementing #161

- adding theme support which works as follows: I've added a Twig loader chain which will first check the defined theme directory for the to-be-rendered templates and if they are not found, they will be looked for in the DEFAULT_THEME directory
- removed myconfig.inc.php (merged with config.inc.php)
- added config.inc.php.new (the file which has to be filled in by the user) so that config.inc.php with actual database credentials can be ignored by git
- added updateUserConfig($userId, $key, $value) to Database.class
- changed way of checking whether Poche is installed (the code is still located in Poche.class.php, but the actual check happens in index.php)

Please test, I have also send a pull request for poche-themes which is compatible with this new change-theme stuff. Note that when you merge, you should change the sub module "themes" URL to git@github.com:inthepoche/poche-themes.git for future compatibility (instead of pointing to my forked version).