fix bug 843068 - When signing up, some users enter their email address in the "Username" field

https://bugzilla.mozilla.org/show_bug.cgi?id=843068

Page: https://developer.mozilla.org/en-US/users/browserid_register

- I've add the placeholder "john_smith" into "Username" input

- I've add the text: "Please enter the name you'd like to display to other users to identify your contributions." under "Username" input.

- I've changed the rule to reject the "@" character in the input and I've deleted the "@" in the error message (now it's "Letters, digits and ./-/_ only."