Updating `workers` module.

Updating `Worker` class. Adding `subprocess_kwargs` attribute to allow
extra arguments to be passed to `subprocess.Popen` call. Updating debug
message (converting ``self.cmd`` elements to strings when writing the
command).

Adding `GenericWorker`, `GeneTreeSimWorker`, and `SequenceSimWorker`
classes. `GenericWorker` extends `Worker` to be as generic as possible
but still `run`able. `GeneTreeSimWorker` and `SequenceSimWorker` are
wrappers around the `genetree_in_sptree_sim` and `simulate_sequences`
scripts from Joseph Heled's `biopy` package.
Adding code to ``examples`` to simulate data.

Adding ``_simulate_example_data`` subdirectory to ``examples``. This
contains working code for simulating example data for tutorials etc.