[simulator] OCaml sim: simulate sys_time

should fix #962
[rotorcraft] datalink: check ac_id of RC_4CH message

should fix at least some of the problems described in #965
[tests] skeleton for testing math lib with libtap

using [libtap](https://github.com/zorgnax/libtap) to create a [TAP](http://testanything.org) (TestAnythingProtocol) producer.
You can also run the test (if already compiled) with prove:
```
prove --exec '' tests/math/test_pprz_math.run
```