#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). 
I never had so much comments on one line :)).

But I like the idea and want to explore it a bit more. Perhaps next week. @agudian seems to be a bit more doubtful though (gathering from his comments above) and likes to have all the checks in his IDE (so using the IDE's on-the-fly code generation to verify the expressions). Perhaps we should introduce a majority vote. Then I'll have the `@BeforeMapping / @AfterMapping` and the expressions :)))))