Fix broken install after ES6 conversion.

[danmakenoise]

The usage of const outside of the module scope in index.js breaks the installation of this package for us, with the following error:

SyntaxError: Use of const in strict mode.

So, I went ahead and fixed it! Here is what I did:

1. Create eslintrc.json to specify eslint to check for ES6.
2. Remove use strict inside of modules, where it is not necessary.
3. Revert index.js back to using var outside of the module scope, where const is not allowed.

[kevinsawicki]

Thanks for this but #28 has been merged which uses babel to precompile the sources files to restore support for node 0.10/0.12 which should address this issue as well.