#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). 
@gunnarmorling , tomorrow I want to look at the current constants and:
1. replace the current expression to constant (as we discussed earlier)
or
2. perhaps keep on using the expression and do it like you propose above: `@Mapping( target = "uom2", expression = "java(\"cm\")")`  (but limited to constants for this release).
or
3. do it perhaps a bit more concise, e.g.  `@Mapping( target = "uom2", expression = "constant(cm)")`  for a String constant or `@Mapping( target = "uom2", expression = "constant(5l)`  for a long constant

Personally I like option 3 because for 2 I assume that the intention is to leave the java code 'as is', while 3 does some extra stuff (like applying type conversion and mapping methods). WDYT?