Fix account creation

**This PR depends on #16 and #17, please merge them before**

Account creation from configuration page is now complete. It seems this
feature actually was never finished, since :

1. it contains a "name" input that does not reflect in database
2. it creates accounts, but they can't be confirmed

Added confirmation action so that created users receive a confirmation
email with a link to confirm their account. Once confirmed, they are
immediately displayed password edition form.

When a maintainer create an account, he's now warned a confirmation mail
has been sent.

Close #13

Details :

* ADD Account#confirmation_token
* ADD generate confirmation_token for new accounts
* ADD accounts#confirm
* ADD confirmation mailer
* REFACTOR send confirmation email on presenters#create
* REFACTOR warns confirmation has been sent when creating account
* UPDATE readme