build pip wheel in wrapper cmake

[figroc]

The original CMakeLists.txt for python installs the lib and py files directly to python site. They are not versioned and the installation dir is affected by CMAKE_INSTALL_PREFIX. While the latter is not necessary bad, the former definitely is and it's bad for redistribution. This PR is partially related to #93.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[figroc]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[cheng-chi]

@figroc @jmr Is there any plan to merge this branch?

[calestyo]

anything new on this?

[jmr]

Can you resolve the conflicts?

[jmr]

CI is failing: https://travis-ci.org/github/google/s2geometry/jobs/726922409#L873

It looks like setuptools needs to be installed first. Can you update README.md with new python build instructions and update .travis.yml to do pip install setuptools?