fix(account): do not set hoodie.account.username for anonymous accounts

This fix also includes

- `hoodie.account.signUp` no longer triggers `account:signin` event
- `hoodie.account.anonymousSignUp` no longer triggers
  `account:signin:anonymous` event
- `hoodie.account.anonymousSignUp` now triggers
  `account:signup:anonymous` event
- `hoodie.account.changeUsername / changePassword` no longer trigger
  `hoodie:account:signin` event, but `changeusername / changepassword`

* * *

This commit was sponsored by The Indie Tech Summit where
200 people will meet to discuss sustainable & ethical
alternatives to corporate surveillance & launch the
Indie Tech Manifesto in Brighton, UK on July 4th, 2014.

http://go.hood.ie/indietech-summit
Merge pull request #269 from hoodiehq/269_anonymous_signup_fix

hoodie.account.username should not be set for anonymous accounts