Changed option for setting the max number of task steps from -s to -z.

[rykerfish]

The script to call tmkit has two conflicting options that use -s. One is for setting the scene file, one is for setting the max number of task steps While there is an alternative form for the steps options, --steps, it is not mentioned in the --help output. I've changed the single character option for steps to -z and added mention of the alternative --steps in the --help output. This PR resolves #16