Added singleton scope concept

The Singleton scope is defined by DI spec and not by CDI spec. But since these specs are closely related; the concept of singleton can be part of cdi plugin.

The concept can come handy to define rules if for some valid reasons @ApplicationScoped is preferred over @Singleton. (http://docs.jboss.org/weld/reference/latest/en-US/html/scopescontexts.html#d0e2061)