Use locale domain `messages` in gettext.js

Was hoping to use `gettext.js` in conjunction with grunt-i18n-abide.

This pull:
* removes the domain `client` and uses the colloquial domain `messages`
* synchronizes what is produced with `abidecompile` - [relevant source](https://github.com/mozilla/grunt-i18n-abide/blob/d5da93cb3ab31b5e95e5c17383afe09cd903b4ae/tasks/abidecompile.js#L72)
* fixes linting errors




The original project that used i18n-abide had two string catalogs, `messages.po` and `client.po`. The server side localization was done with `messages.po`.

This is a great pull request, but let's make the key configurable, since different projects will want to use `messages`, `client` or some unforeseen name.

The default value should remain `client` to avoid breaking existing clients and then via configuration you could choose to use `messages` instead.