Support installation on Windows

[abcoathup]

Support installation on Windows.

Currently npm install on Windows results in the following error:
'ln' is not recognized as an internal or external command, operable program or batch file.

OpenZeppelin Network.js uses preinstall to create a symlink for React.
ln doesn't exist on Windows.
https://github.com/OpenZeppelin/openzeppelin-network.js/blob/master/package.json#L14

Alternatives are to use Windows Subsystem for Linux or if using Windows natively, install using Git Bash terminal.