Add requirements.txt to sdist, remove README.md from data_files

[charles-dyfis-net]

Fixes #47.

• requirements.txt being missing from the source tarball meant that this package was only possible to install from the wheel, not from the sdist; one can reproduce the problem by using --no-binary :all: as an argument to pip.
• Adding README.md to data_files meant that it was installed into site-packages instead of only being added to the source tarball; being at the root, it conflicted with any other Python package trying to install a README.md when using a package manager that tracks such conflicts.