Added noConflict() so multiple versions can be included in the same context safely

Hey @dankogai,

First off, thanks for your Base64 library! It's working really well for me.

I've added a `noConflict` method so that multiple versions/instances of Base64 can be included in the same context safely. I'm currently using it in a third-party JavaScript library that developers can include on their site. I don't want the Base64 version I'm using to conflict with a version already being included by the developer. Other major libraries like Underscore and jQuery include a `noConflict` method if you'd like to compare implementations.

I've only made changes to the source. I haven't minified, updated the README, or added tests. I can add tests if you'd like — I'm just not sure which test file to add to?

Let me know if you have any questions or comments. Thanks!