#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). 
Right, for 2. the idea would be to leave the code as is, assuming Java is used as "expression language". For other languages we'd have to apply a conversion into Java code. I'm no big fan of 3 as it is kind of assymetric ("constant" is no language as opposed to "java" or "spel"). Having a separate attribute `constant()` (which is subject to conversions) and - in a subsequent release - `expression()` for more advanced use cases still seems to me to be the best approach.