Plugin with settings

First attempt at addressing #4

This creates a basic plugin which adds a single settings page for adding the various Twitter API keys and the transient expiration time. I haven't tested it 'cause I'm kinda tired :)

I kinda think we should do this differently anyway. At the moment, there are effectively four different external libraries in this plugin;

* FooTweetFetcher itself
* TwitterOAuth
* OAuth
* WordPress SimpleSettings

So rather than turn this existing library and repo into a plugin (maybe) we should have a new plugin which  simply pulls in the above four libraries using [Composer](http://getcomposer.org). The benefits being;

* The original FooTweetFetcher is maintained as-is, avoiding screwing up anyone's existing usage of it.
* Folks who want to use the new plugin can do so using composer
* We can use a "build" branch in the plugin repo to create the version for submission to WordPress.org

The caveat with the above is that it would require one change to the FooTweetFetcher class - the API key settings would need to be filtered, so that the plugin can actually change them based on the user-entered settings.

How's that sound?