refactor pysift as a library with install

[normanb]

@rmislam As I was working through the code I refactored the library slightly and added a packaging script (setup.py). If you are interested in this PR then I can expand further and add tox for testing as well as refactoring the code a little to include more testing and modularity.

Thank you for writing this and the excellent tutorial on the SIFT algorithm.

[rmislam]

@normanb Wow, thanks a lot! This is extremely helpful. Yes, I'm very interested in this PR, and please by all means expand further. Truth be told, I don't have too much time these days, but if I can help you out somehow, please let me know.

[normanb]

@rmislam thank you. I am going to make the library callable by breaking out the functions where I can and so that the code can be ran in parallel to increase performance. I can work in this branch with separate commits so you can clearly see the changes. I understand that the code needs to be readable enough for people following your tutorial.