Created annotation processors that verify correct usage of the ErraiUI annotations. Disabled by default.
Merge parts of Uberfire Security into Errai. Details below.

* Imported UberFire's Role and Subject (now User) types; standardized Errai security on these

* Centralized the role membership checking logic onto User; added unit tests for it

* Refactor ActiveUserCache into SPI.

* Create ANONYMOUS user constant to use instead of null when no user logged in.

* Switch to cookie storage of user info so that we can unify with UberFire's form based login

* Use cookies instead of local storage; add CookieEncoder class into shared package.

* Make todolist demo user an errai security User.

* Delete Identity class (it's redundant; Caller<AuthenticationService> can be used).