#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 may be missing a piece, but what is the advantage of this over the following (which should already work today, provided the required conversion methods exist):

This is purely alignment of notation with the reverse mapping. I would like deprecate / remove the existing (that you pointed out).

> I think what we really need is support for referencing nested properties as this (which wouldn't require any custom conversion methods to do the de-composition of the source object):

Ok. But that does not fix the major part of this work: the reverse problem.

Furthermore, have that 'dot' notation already.  Right? (ch 3.3 in the Mapstruct documentation). Only there the first part means the name of the argument iso a property. I think that (is already :) / would) be confusing.