faster install process with prebuilt binaries

[DavidVujic]

Fixes #250

Description

The source code will include prebuilt binaries for Mac OS X (darwin, x64) and Windows - for node 12 and 14. This will simplify and fasten the install process a lot.

Especially for Windows users that no longer will need to have CMake or C++ build tools installed. For Mac OS X users, the install process will be much faster.

In short: npm install will not trigger a Native AddOn build, because the binaries are already there. However, builds will still be run for systems that does not match the binaries.

How Has This Been Tested?

• Travis CI build OK
• Mac OS X: installed the library - verified there was not builds - and ran an example project connecting to a ZooKeeper server (hosted in docker).
• Windows 10: installed the library - verified there was not builds - and ran an example project connecting to a ZooKeeper server (hosted at the local machine). A virtual Windows machine running in Azure.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I have read the code of conduct.
• I have read the contributing guide.
• I have updated the documentation accordingly (if applicable).

[DavidVujic]

The package will be heavier, but I think it is worth it.

Also, I am thinking about dropping Node 12 prebuilds during 2021.