Add ESLint as javascript tool

[rjkip]

As originally submitted by @DRvanR:

http://eslint.org/

Depends on #93

[rjkip]

Scato suggests using the Airbnb eslint preset in #76

[rjkip]

Make sure ESLint is configured to check for sensible cyclomatic complexity. Related JSHint issue #18

[rjkip]

We could also opt for something like https://github.com/jlongster/prettier. Why not actually reformat to a standard, rather than checking and requiring developers to fix it themselves?