mention supported OSs in the README
make sure merge prefixes for git are always forward-slash-separated

Summary:
Git doesn't expect backslashes in tree merge prefixes, but on Windows we
were incorrectly using those. That got things very confused when
anything had an import path with slashes in it. Use pathlib to make sure
we always get a posix path for this.

Fixes https://github.com/buildinspace/peru/issues/60.

Test Plan: New unit test that was previously failing on Windows.

Reviewers: sean

Differential Revision: https://phabricator.buildinspace.com/D136
use entry_points instead of a pre-written script

Summary:
This was the recommendation of
https://packaging.python.org/en/latest/distributing.html, but I unwisely
ignored it back when I wrote our setup.py. It turns out that getting a
launcher script to work properly on Windows is tricky. The
`entry_points` field generates a platform-appropriate launcher script at
install time, so we get Windows for free.

Previously our launcher script checked the version of Python and printed
a helpful error if it was too low. Because our main.py has `yield from`
syntax in it, printing such an error message isn't possible; we'll crash
with a syntax error first. We could create another file within the
modile (or maybe an __init__.py) to support this, but for now I'm just
dropping the feature.

Test Plan:
Manually played with peru.sh to make sure it still works. Ran
`pip install .` to check that the generated script works. Also tested
that on my Windows VM, and it works great. (It generates an exe file, in
fact.)

Reviewers: sean

Differential Revision: https://phabricator.buildinspace.com/D135