Create a new entity. Simple
entities can be constructed by passing a name, value to the constructor; this creates
a generic, plain entity reference. For anything more complicated, you have
to pass a Source to the constructor with
the entity definition, or use the accessor methods. WARNING: There
is no validation of entity state except when the entity is read from a
stream. If you start poking around with the accessors, you can easily
create a non-conformant Entity.