Update server requirements in setup instructions

- Specify the minimal required patch version of PHP 5.3
- Add the mcrypt PHP extension, because Laravel wouldn’t run without it
- Make it clear that Apache is not a hard dependency but that other types of servers such as nginx would also work as well
Update instructions about app/storage permissions

777 is overkill, giving write access is enough.
Merge pull request #92 from miclf/beta

Update setup instructions