Enable boolean values for keys

Fixes https://github.com/yola/plom/issues/2

If a user wanted to set a key to have the boolean false value, it would not work as expected due to the conditional logic.

    Plom.prototype.set = function(key, val) {
      if(val) {
        this.data[key] = val;
        return this;
      }

      this.data = key;
      return this;
    };

`val` would be false, it would skip setting that key to false, and instead set the entire data object to the input key name. By checking if the value is not `undefined`, the conditional now correctly determines if there is a `val` or not.
What if you want a value to be `undefined`?

I'd try looking at `arguments` or checking the type of `key`. If `typeof key === object`, then you can assume the user intends to set the value of the PLOM to that object, because in javascript objects cannot be object keys.