#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). 
Btw. your examples would look like this using the Java code approach:

```java
    @Mapping( target = "longWrapperConstant", expression = "3001L"),
    Target sourceToTarget(Source s); 

    @Mapping( target = "timeAndFormat", expression = "new TimeAndFormat(s.getTime(), s.getFormat())"),
    Target sourceToTarget(Source s); 

    @Mappings( {
        @Mapping( target = "uom", expression = "\"cm\""),
        @Mapping( target = "value", expression = "value"),
    } )
    PreDrilledDepth sourceToTarget(BigDecimal value); 
```
Not so bad I'd say :)