Different separator support added

Separator can be changed now to emulate windows-like (or any other) registry behavior.

    $a = new Registry;
    $a->separator = '\\';
    $a->set('Foo\\Bar', 'mykey');

It also can be used when dots should be supported in keys;

    $filehistory = new Registry;
    $filehistory->separator = '/';
    $filehistory->set('myfile.json', '2014-10-10');
    $filehistory->set('mydir/myfile.xml', '2014-10-09');

Also added support to load arrays made by flatten() method. Now the registry can be safely stored in database and then loaded from it.