Add package setup script to support pip install (#20)

[li-plus]

I make some necessary changes to release the package, as described in issue #20. Detailed changes are

• A setup.py script to support pip install & an __init__.py for package import management.
• The package includes command-line tools closed-form-matting and solve-foreground-background, which will be installed into user's binary folder.
• Remove pytest for Python 2.7 in CI, because the latest opencv-python package does not support Python 2.7 and will fail the CI.
• Some improvements in README.

Before merging this PR, you could install the package by running

pip install git+https://github.com/li-plus/closed-form-matting.git

And maybe do some testing, though all tests are passed in Travis CI.

To publish this package to PyPI, please follow this tutorial https://packaging.python.org/tutorials/packaging-projects/

[MarcoForte]

Thanks @li-plus !, looks great to me. I'll publish it to PyPI