Add a simple port example

[artob]

This pull request adds a basic example of interprocess communication between Elixir and Python scripts using an Erlang port and the Erlang external term format.

Running examples/test.exs will spawn examples/port.py as a port subprocess that will echo back any Erlang term sent to the port, in the examples/test.exs example the single-atom tuple {ping}.

[okeuday]

@bendiken Thank you for the contribution! Can you please tweak it to use nouse_stdio in the erlang:open_port/2 settings, and then use os.fdopen(3) and os.fdopen(4) in the python code without having stream have a default argument in the functions that take stream. That should help simplify the example. Then, the python try/except can be removed to make sure errors can output normally. Also, please indent the python with 4 spaces, to make that a more typical format (2 spaces is typical in elixir, so the elixir indent is good).

[artob]

@okeuday Thanks for the feedback. I've applied the changes you requested. You were right that nouse_stdio simplified the example nicely. Hopefully this is good to merge now?

PS. I've tested the example with both Python 2.7 and Python 3.5. (Elixir 1.3 in both cases.)

[okeuday]

@bendiken Looks good. Thanks for adding this example!