Expose locale options

[jbt]

There's currently some very basic forward-thinking provision in makeFormatter.js for locale support. Currently just for day of week stuff, but could quite easily also be used in the L and l tokens.

At the moment, the locale argument in makeFormatter is hardcoded to null everywhere it's used. Shouldn't be too hard to expose that, probably via speedDate.locale(obj) which would return a new instance of speedDate with its own formatter cache.