Resolves #23 (Class level is-accessible doesn't work)

Resolves issue #23 (Class level is-accessible doesn't work) at https://github.com/DozerMapper/dozer/issues/23 . Logic:

Both class A and B declared accessible: Map from matching fields in A to fields in B
Only class A declared accessible: Map from matching fields in A to matching properties in B
Explicit mappings (via @Mapping or XML) override these default is-accessible="true" mappings