#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). 
That being said, I think there may be a viable solution via the "expression" attribute, e.g. like so:

    @Mapping(target="timeAndFormat", expression="new TimeAndFormat(time, format)")

In other words, `source` should be used for plain mappings (including references to nested properties to cater for the common case of "flattening" a source hierarchy), whereas I envision `expression` to be useful in more complex cases such as the one at hand. For this we'd have to agree on a syntax to be used in `expression`,  so far I considered plain Java code, only properties would have to be resolved and modified into "proper" getter references.