Fixed footer.php for greaves

Outdated call to function Admin URL generate an error message:

    UNCAUGHT EXCEPTION
    Call to undefined function admin_url()
    
    ORIGIN
    
    themes/greaves/footer.php on line 14
    
    TRACE
    
    #0 [internal function]: System\Error::shutdown()
    #1 {main}

Replacing on line 14 of `footer.php`:

    <?php echo admin_url(); ?>

With:

    <?php echo base_url('admin'); ?>

Correct syntax taken from default theme.

And in fine, adding a `.gitignore` file for not messing the repository