greater flexibility and protection when exporting timezoneJS

The current method for exporting doesn't play well with some other less-friendly libraries. This can cause things like infinite recursion and all sorts of pain.

Specific library causing me problems: https://github.com/bytespider/jsOAuth
jsOAuth does this:

    var exports = exports || this;
    exports.OAuth = (function (global) {
    ...

In the browser, the problem here is that `exports` is now `window`. timezoneJS then comes along and does this:

    timezoneJS = exports; // equivalent to `timezoneJS = window;`
    ...
    timezoneJS.Date = ...; // equivalent to `window.Date = ...;`

Then, any time you try to create a timezoneJS.Date object it invokes `new Date(...)` which is equivalent to calling `timezoneJS.Date(...)` -- you see the problem this creates.

So I adapted the export pattern used by timezoneJS to follow the pattern used by underscore.js which also accounts for `module` and the problems went away. The tests all still pass for me in node@v0.10.12 and it also works in harmony with jsOAuth and other modules that blindly muck around with the global `exports` object.