Robustly install snowboy

[jacobtomlinson]

Currently the install requires snowboy to be compiled for the current system and python version separately to the installation of opsdroid audio.

Snowboy's python wrapper isn't a python module (doesn't have an __init__.py) so once you build the project you also have to touch __init__.py in the swig/Python directory. You must then rename swig/Python to snowboydetect and put it somewhere on the python path.

Also if you're building the project for python3 you need to update the Makefile in swig/Python to call python3-config instead of python-config.

All of this is currently captured in scripts/install_snowboy.sh to be used during travis builds but this also needs to be implemented in a robust way for users installing opsdroid-audio.