Make options parameter optional

This allows the module to be used like so:
ghpages.deploy(basePath, callback)

lib/index.js

@@ -50,6 +50,11 @@ function getRepo(options) {
  * Push a git branch to a remote (pushes gh-pages by default).
  */
 exports.publish = function publish(basePath, config, done) {
+  if (typeof config === 'function') {
+    done = config;
+    config = {};
+  }
+
   var defaults = {
     add: false,
     git: 'git',