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

ericf/intl-messageformat,rlivsey/intl-messageformat,yahoo/intl-messageformat,ericf/intl-messageformat,rlivsey/intl-messageformat,mattikl/intl-messageformat,ericf/intl-messageformat,jeffvan576/intl-messageformat,rlivsey/intl-messageformat,yahoo/intl-messageformat,mattikl/intl-messageformat,jeffvan576/intl-messageformat,yahoo/intl-messageformat,jeffvan576/intl-messageformat,mattikl/intl-messageformat