Fix #17 - fail_now should result in exit code 1

[johscheuer]

This PR ensures that fail_now and all other assertions that use assert.AssertionError results in an exit code 1

[jsleeio]

looks ok to me, though I haven't personally tried the function you're referring to

thanks for sharing!

[vgramer]

@garethr can you merge and release this PR ?

[yastij]

@garethr - can you release this patch when you have some bandwidth ? Thanks !