Split the SafariDriver's injected and page scripts into several modules so
we can avoid loading large amounts of JS before it is needed. The
performance improvement is most noticable with pages that contains lots of
frames that the driver may not interact with.  The included benchmark runs
in 73 ms without the SafariDriver installed, 1250 ms using the driver
without modules and 518 ms using modules. The modules in this change are
pretty rough, so it's probably possible to squeeze out further improvements.