made NetTxRx.check() a bit better

added detecting for RX-OK and TX-OK positions (sick and tired from hardcoded values) and check that fields are numeric

https://travis-ci.org/nnugumanov/yandex-tank/builds/31090916 - successfully built
Merge pull request #2 from nnugumanov/patch-1

made NetTxRx.check() a bit better