Added eslint and mocha test.

[justin-lau]

This PR does two things:

1. Added linting to the package to maintain a persistent coding style. The eslint config is copied from gobble core. The concat() function is removed from index.js because it isn't used anywhere.
2. Added basic structure for testing with mocha. I also added a simple test case that currently fails due to Require fails at runtime with "Cannot find module ..' #6.

I try to stick with the coding style and file structure used by @Rich-Harris as much as possible, but the eslint rule doesn't check everything.