Make email address less restrictive and add icon to show when it's correct

Fixes #414
I used androids default android.R.drawable.presence_online and android.R.drawable.presence_offline
because they fit very well. The online one is just green and suggests a correct structure and the offline is gray with a faint X on it. Using a red cross here would suggest to the user that his email/icq/whatever address isn't accepted so I avoided it. Also when the mail is empty i remove the drawable completely.
Tested on android 2.3.3 and 4.1.3 and works as expected.