[FIX,TEST] fixes failing SequestOutfile_test when running with libc++

 * libc++ seems to handle self assignment of std::map a bit differently which leads to a failing test
[FIX] MRMDecoy uses std::random_shuffle which works well with VS and libstdc++ but not libc++

 * libc++ has a different implementation of std::random_shuffle which leads to a different output
 * the c++ standard actually does not define the exact behaviour of std::random_shuffle so it seems to be our problem to handle it
 * I replaced the call to std::random_shuffle with the implementation from libstdc++
[DOC,NOP] added another comment giving some more details regarding the swapping intervals
Merge pull request #919 from aiche/fix/libc++-test-fixes

[FIX] libc++/macosx test fixes