Add os for tests

[SimonBaeumer]

Add a field which defines on which os a test can be executed, i.e.:

tests:
    echo hello:
        os:
        - darwin
        - linux

or maybe this is even better:

config:
    os: linux # Define default os
tests:
     print hello:
         cmd:
             osx: echo hello osx
             windows: echo hello windows
             linux: echo hello linux