Add `seeSourceEquals()`

[ThomasLandauer]

When asserting "small" pages, it sometimes can be handy to assert that the page source is equal to an expected string. REST module has a nice method for that: seeResponseEquals().
I'm suggesting to include the same functionality into WebDriver, calling it seeSourceEquals().

I suggested this for lib-innerbrowser as well: Codeception/lib-innerbrowser#14