Implemented YelpClient.

Example usage.

    var Yelp = require('./yelpapi').YelpClient;

    var authConfig = {
      OAuthToken: 'Token',
      OAuthTokenSecret: 'TokenSecret',
      key: 'Consumer Key',
      secret: 'Consumer Secret'
    };

    var test = new YelpClient(authConfig);

    test.searchRestaurants({
      sort: '2',
      radius_filter: 1000,
      location: '944 Market Street, #8 San Francisco, CA 94102'
    }, function(err, data, res){
      console.log(err, data)
    })