#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):

    @Mapping( source = "timeAndFormat", target = "time" )
    @Mapping( source = "timeAndFormat", target = "format" )

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):

    @Mapping( source = "timeAndFormat.time", target = "time" )
    @Mapping( source = "timeAndFormat.format", target = "format" )