Added `page.goBack()` and `page.goForward()`

Quick fix, just added to the wrap the two missing methods
`page.goBack()` and `page.goForward()`.

I guess there is no need for testing.

I am not sure about the methods `page.go(index)` hence I did not add it.
Merge pull request #92 from pongells/master

Added `page.goBack()` and `page.goForward()`