Remove dist from repo

[zcei]

Dist files are only needed for pushing to npm.
In order to keep commit diffs as small as possible remove dist/index.js and add dist to .gitignore.
Adds .npmignore to not ignore dist folder while publishing.

[itaylor]

Let me think about this one. The thing I like about having dist checked in is that you can get the exact same code that is in NPM by checking out the repo.

[zcei]

How about providing a README section on how to build the code?

Or even some npm run build script.
Only drawback would be to include babel/babel-cli inside of the project, so we don't rely on them having the babel-cli installed. That's why I didn't include this approach here.

[zcei]

As this discussion came up at work today, here the sentence with which I argued it against my colleagues:

It's called git-scm - source control management. Your compiled/built code is an artifact, not actual source code

Just thought I drop this note here, so you get why I created the PR :)