Replace deprecated view helper

The "renderFlashMessages" view helper has been marked as deprecated
in favor of "flashMessages" a while ago.
Also use the "div" renderMode to have TYPO3 styles flash messages
by default
Update CSS for the TYPO3 6.0 login mask

The styles have been changed, the provided custom style sheet should
include those changes into its own styles. Otherwise the layout is
broken, eg: the installation name is under the username input field.
Merge pull request #5 from Trenker/v6-login

V6 login