Improve closed_form_matting API. Add CLI. Add tests + CI

[merofeev]

This PR adds multiple things:

• Enables construction of Matting Laplacian for only unknown part of image (seems to save a lot of memory and time for large images)
• Improves closed_form_matting.py API. It can be used now with trimap input, scribbles input and the most general prior map input.
• Adds CLI to closed_form_matting.py
• Adds tests and Continuous Integration config
• Updates README.

P.S. Please consider (if you haven't already) create account at travis-ci.org, enable CI for this repo, and add travis badge to README

[![Build Status](https://travis-ci.org/MarcoForte/closed-form-matting.svg?branch=master)](https://travis-ci.org/MarcoForte/closed-form-matting)

[MarcoForte]

Thanks, all great improvements!