#65 support for nested properties

Finally, after a lot of iterating hence-and-forth. Here's the solution for nested properties as discussed in #65.

Some points addressed:

1) (Error)Checking of the source side has been moved to (construction-time) of the `@Mapper`
2) `BeanMappingMethod` has been restructured. It first creates a  'todo' list of target properties, and than slowly empties the list by first handing the defined `@Mappings` that act on a target property, and then handles the remaining properties based on name.
3) Finding a suitable name candidate is handled completely in `BeanMappingMethod`
4) `SourceReference` contains all required information on the source part of the mapping distilled from the source information (so method and mapping). 
5) `BeanMappingMethod#build` became to large. It has been partially refactored, although perhaps not as much as I desired. An `getPropertyMapping` should perhaps be made per `TargetAccessorType` iso squeezing them all in the same method. But that's for a next refactoring :).
6) I used the existing `ForgedMethod` mechanism to handle the nested properties in an isolated fashion. That leaves room for extension (e.g. list and map support).
7) I handled the parameter-only (so no source bean) case as well.

> Btw. all the nested null checks (I didn't really think of that issue before) make me think that we should add support for Java 8's Optional type at some point. It may be used in models to express optional members.

Hmm. Someday I need to get more into Java 8 :).