Validate API key

Note, I couldn’t figure out how to run the tests to verify that my
change didn’t break anything. See
gomfunkel#37

lib/mailchimp/MailChimpAPI_v2_0.js

@@ -1,6 +1,6 @@
 var http = require('http'),
-    request = require('request'),
-    helpers = require('./helpers');
+	request = require('request'),
+	helpers = require('./helpers');
 
 /**
  * MailChimp API wrapper for the API version 2.0. This object should not be 
@@ -15,6 +15,11 @@ function MailChimpAPI_v2_0 (apiKey, options) {
 	if (!options) 
 		var options = {};
 
+	// API Key must look like abc123-us2 (where us2 is the data center)
+	if (!apiKey.match(/^[a-z0-9]*\-[a-z0-9]*$/)) {
+		throw new Error('Unrecognized API key format: ' + apiKey + '. should look more like abc123-us2.');
+	}
+
 	this.version     = '2.0';
 	this.apiKey      = apiKey;
 	this.packageInfo = options.packageInfo;