Login using email instead of username

More and more services allow the user email to be used for login-in. It could be very interesting to have this in order to align XWiki with other web services.

It would imply at least to :

• Have the email unique
• Update the "password lost" page
• Update the subscription forms and process

Since all companies may not want this feature, having it as an option could be to think of but it would imply :

• Having a process to switch from one mode to the other