/**
        * @constructor Animate
        * @param {HTMLElement} el the element we want to animate
        * @param {String} prop the CSS property we will be animating
        * @param {Object} opts a configuration object
        * object properties include
        * from {Int}
        * to {Int}
        * time {Int} time in milliseconds
        * callback {Function}
        */
      function Animate(el, prop, opts) {
        this.el = el;
        this.prop = prop;
        this.from = opts.from;
        this.to = opts.to;
        this.time = opts.time;
        this.callback = opts.callback;
        this.animDiff = this.to - this.from;
      }

      /**
        * @private
        * @param {String} val the CSS value we will set on the property
        */
      Animate.prototype._setStyle = function(val) {
        switch (this.prop) {
          case 'opacity':
            this.el.style[this.prop] = val;
            this.el.style.filter = 'alpha(opacity=' + val * 100 + ')';
            break;

          default:
            this.el.style[this.prop] = val + 'px';
            break;
        };
      };

      /**
        * @private
        * this is the tweening function
        */
      Animate.prototype._animate = function() {
        var that = this;
        this.now = new Date();
        this.diff = this.now - this.startTime;

        if (this.diff > this.time) {
          this._setStyle(this.to);

          if (this.callback) {
            this.callback.call(this);
          }
          clearInterval(this.timer);
          return;
        }

        this.percentage = (Math.floor((this.diff / this.time) * 100) / 100);
        this.val = (this.animDiff * this.percentage) + this.from;
        this._setStyle(this.val);
      };

      /**
        * @public
        * begins the animation
        */
      Animate.prototype.start = function() {
        var that = this;
        this.startTime = new Date();

        this.timer = setInterval(function() {
          that._animate.call(that);
        }, 4);
      };


      /**
        * @static
        * @boolean
        * allows us to check if native CSS transitions are possible
        */
      Animate.canTransition = function() {
        var el = document.createElement('twitter');
        el.style.cssText = '-webkit-transition: all .5s linear;';
        return !!el.style.webkitTransitionProperty;
      }();