Add redirect counter to RackTest::Driver

In application I'm working on right know, we've monkey patched RackTest driver to keep track or redirects count. We have few cucumber steps which rely on it as an easier way to have common step checking if redirect happened. Alternative solution was checking `Location:` url but it's different for each case, thus just checking if redirect happened is was more better and applies to all cases.

This PR adds redirects counter to `RackTest::Browser` and simple proxies in `RackTest::Driver` for easier retrieval. (we've also patched both in out app)