Remove use of `indexOf()` when adding locale data

`Array.prototype.indexOf()` is an ES5 method we were using when it
wasn't necessary. This remove its usage, and improves the error
handling when adding locale data and resolve locales.

Fixes #76
Use `hop` instead of `in` for safety
Merge pull request #77 from ericf/rm-indexof

Remove use of `indexOf()` when adding locale data