Snort integrate: Add the snort lib supporting snort integration.
Add an sample application simple_switch_snort.py which can dump alert message.

When there is a Snort alert message, Ryu will receive a EventAlert's event and
you can easily define the event handler in the method which using ‘set_ev_cls’ decorator.

The simple_switch_snort.py can install a flow
that mirror packets to the snort network interface card on port 3 (by default).

There are two options that sending alert message to Ryu.

1. Ryu receives Snort alert message via Unix Domain Socket on same machine.

2. Ryu receives Snort alert message via Network Socket on different machines.

More detail see /ryu/doc/snort_integrate.rst

Signed-off-by: Che-Wei Lin <linton.tw@gmail.com>