implement fix for issue #139

This adds a mutator that replaces non-void method calls with one of its
parameters of the same type as the return value in case such a parameter
exists.

Thanks to @smandel for showing me how to use ASM and working on a first
draft of this with me.
test for case when return value is not used
refactoring
test that mutator is listed
update copyright information
rename mutator to "argument propagation"
Merge branch 'UrsMetz-replace-method-with-parameter-of-matching-type'
mark argument propagation mutator as experimental
update readme