#113 renaming unit test, adding unit test, implementing solution

For quick understanding :) please look at tests:
1. `org.mapstruct.ap.test.multiple.manytoone.SourceTargetMapper`
2. `org.mapstruct.ap.test.multiple.onetomany.SourceTargetMapper`

This is a first attempt. This solves a considerable part of the current issues I have. 

About the solution, I need to add unit test for the error situations that are introduced. But have a look and see if this makes sense. If I need other tests as well ,let me know.

I'm thinking of deprecating (or removing, suggestions?) the current multiple sources mechanism (so, where a `@mapping` to the same target is allowed more than one time in the `@Mappings`) in favor for the newly introduced comma separated mechanism (in order to align with many-sources-to-one-target). 
> >  To be honest, I prefer the method with the @AfterMapping @BeforeMapping because of its simplicity on the cases were I require it.
> 
> I see. @agudian will like to hear that. Maybe there is something to this after all ;)

Maybe? Dude, that hurts. But at least you're making progress on catching up on the rest of us :wink:

On the topic, my feeling is that we shouldn't allow stuff that we can't check at generation-time and that we can't transform to plain java code. So what ever we do with such expressions should be parsed, checked and transformed to simple java code.