Added prepare script to package.json

[dudaerich]

Since npm@5.0.0 the prepare script is called
when the library is installed from github url.
Aim of the script is to build distribution
files.

See npm/npm#3055

[j-rewerts]

Here is NPM's docs on it. This seems to work for me. It automatically builds the project when I run npm install, which is convenient. I'm still not 100% on how this impacts our build process though, since we support Node 4, which doesn't run this.

I think we should probably wait until we get our tests passing, then rerun our CI for this PR.