cfme login tests work if the browser is already logged in

Also caught a bug on is_displayed: `browser()` (a WebDriver) has no
`is_displayed` method.
force_navigate now passes args (like context) back to go_to
Merge pull request #419 from seandst/login

cfme login tests work if the browser is already logged in
Merge pull request #420 from seandst/force-nav-passes-args

force_navigate now passes args (like context) back to go_to