Add support for .translatable() marker functions

This is meant to address Issue#3.

This supports languages like JavaScript where it would not be a good idea to
implement translatable() as a global since it is common to have global
collisions. So an implementation like this:

Locale.Maketext.Utils.MarkPhrase.translatable = function(str) {
    return str;
};

Could be used to harvest phrases from JavaScript like:

var defaultKey = Locale.Maketext.Utils.MarkPhrase.translatable("You have [_1] of [_2] items selected.");
var key = userOverride || defaultKey;
var localized = LOCALE.maketext(key);