Added ability to set next, skip, and previous button text via new .setoptions({}) method

This is intended to fix issue #10

This addition allows for the labels of the buttons to be set by calling:
```javascript
introJs().setoptions({nextLabel: 'This Way', skipLabel: 'Go Away', prevLabel: 'That Way'}).start();
````
Or, individually like
```javascript
introJs().setoption('nextLabel', 'This Way').start();
````

This adds two methods on the public object:
```javascript
.setoptions({key: 'val', key2: 'val2'})
.setoption('key', 'val')
````

And one method to the private object:
`_mergeOptions(obj1, obj2)`