#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). 
> But I like the idea and want to explore it a bit more. Perhaps next week. @agudian seems to be a bit more doubtful though (gathering from his comments above) and likes to have all the checks in his IDE (so using the IDE's on-the-fly code generation to verify the expressions).

Yea, I still think that having all type-information and the control over formatting and so on would be great as well. Besides, It would be fund fiddling around with a parser generator again. I used [SableCC](http://sablecc.org/) several times now and it is really great (gives good error messages with precise positions if something can not be parsed, creates an AST that can easily be traversed with a visitor-pattern, etc).
But I can also understand the desire of you two to create something powerful with much less effort. As for the imports, we could add another optional parameter that allows to specify the types to import. We could also extract that expression and the import attribute into a new annotation type ```@Expression(..) that encapsulates all that (and which may allow us to be more flexible there).

> Perhaps we should introduce a majority vote. Then I'll have the @BeforeMapping / @AfterMapping and the expressions :)))))

Hehe, interesting idea. I wouldn't be against it, but I'm also happy with Gunnar being the [BDFL](http://en.wikipedia.org/wiki/Benevolent_dictator_for_life) around here - I'm still confident that we'll get our PR #14 sooner ~~or later~~ :wink:.