Remove dist from source and add npmignore file

[Thomas Lindner]

The goal of this is to remove dynamically generated files from source control.

Items we need complete

• Update .gitignore to ignore dist/
• Remove dist/ from source control
• Add an empty .npmignore file (This will allow us to include the dist directory when publishing the node module, but exclude it from source control.)
• Remove dist references in README.md and CONTRIBUTING.md

[Chris Rebert]

As with Bootstrap, we include dist/ for the convenience of users, so as to not require them to setup their own transpilation toolchain when using the package in simple cases.