Make test browser implicitly available

When writing integration specs using a browser, it is hard to reuse code because you need to pass the browser with each method call. This changes therefor makes the test browser reference implicit.

Now I can write code like:
```scala
def login(username: String, password: String)(implicit browser: Testbrowser) = ...

"My admin area" should {
  "be available for admins" in new WithBrowser {
    login("admin@domain.com", "foo")
    browser.url must beSome("/admin")
  }
}
```