Add local timestamp functions (fix issue #42)

So it implements the function we talked about in #42 with mock support (see the tests for info on how to use this). I did not implement it for uc/OS-III yet, because I don't have a working system with me now. @nuft could do it maybe ?

I would prefer to have the ucOS implementation before merge, to avoid splitting the functionality list between the test version and the real version.