Move ext-intl from a requirement to a suggestion

Replaces #45.

I've tried to capture all places where the intl extension is required and throw an exception, and skip those tests. The only that I haven't covered is the Locale subclass, is there any reason that this extends the base Locale class? (Looks to me like it's just there for the static method.)

I'll change this commit once or twice to try and trigger a Travis build without the Intl extension.