Improve split performance

added small improvement of neighbour helper
added a unittest to do performance testing - this test is not executed by all_tests
improved hashfunction (http://computinglife.wordpress.com/2008/11/20/why-do-hash-functions-use-prime-numbers/)
completely new splitdetector

I don't notice performance issues with splitdetection of very fast bullets or rockets anymore (on windows - release build without debugger)

#167