redirect /app.php to prevent duplicate content

Improved version of #497. This time it's compatible with Apache < 2.3.9 by not using the END flag. It also features default compatibility with apache aliases (inspired by [Zend](https://github.com/zendframework/ZendSkeletonApplication/blob/master/public/.htaccess)). And it improves the rewriting performance for the homepage by preventing the process to apply to each DirectoryIndex file.

Now I also could also make the startpage work when mod_rewrite is not available.

I tested it thoroughly and also debugged apache rewriting to see what's happening.
It was quite hard to find out how to achieve all that. [This resource](http://stackoverflow.com/questions/7798099/how-to-block-multiple-mod-rewrite-passes-or-infinite-loops-in-a-htaccess-cont) helped quite a bit.

May be good if someone else can confirm all works.

Example behavior for each scenario:

With mod_rewrite:
- `/` -> internally rewritten to use front controller (`app.php`)
- `/path` -> internally rewritten to use front controller
- `/app.php/path` -> external permanent redirect to `/path` to prevent duplicate content
- `/app.php` or `/app.php/`-> external permanent redirect to `/` to prevent duplicate content
- `/app.phpF` stays as-is

Without mod_rewrite but with mod_alias (default module):
- `/` -> external temporary redirect to `/app.php/` so it's explicit and all generated links on the startpage also work with the base path prepended
- `/path` -> stays as-is and would probably return a 404 (we cannot safely and easily redirect with mod_alias to `/app.php/path` because we don't know if the request references a real file like CSS)
- `/app.php/path` -> stays as-is and works because the front controller is given
- `/app.php` -> stays as-is and works because the front controller is given

In an aliased environment with mod_rewrite:
- `/myproject/` -> internally rewritten to use front controller (`app.php`)
- `/myproject/path` -> internally rewritten to use front controller
- `/myproject/app.php/path` -> external permanent redirect to `/myproject/path` to prevent duplicate content
- `/myproject/app.php` -> external permanent redirect to `/myproject/` to prevent duplicate content

In an aliased environment without mod_rewrite:
- `/myproject/` -> stays as-is and works because it uses the DirectoryIndex definition (but the links will not work)
- `/myproject/path` -> stays as-is and would probably return a 404 (except there is such a file)
- `/myproject/app.php/path` -> stays as-is and works because the front controller is given
- `/myproject/app.php` -> stays as-is and works because the front controller is given