handle http auth

[reepush]

Finally got it working for both Chrome and Firefox!

• In emulation mode callback for mooltipass.device.retrieveCredentials didn't execute that's why there was no auth popup.
• Stuck with a problem that it's impossible to get Authorization header while we intercept response. But we can create our custom popup and handle http authorization by ourself. I used window.prompt now, will add popup later.
• Firefox doesn't execute callback because of the asyncBlocking setting. It's not supported and I will see the differences between Chrome's asyncBlocking and Firefox's blocking.

[reepush]

Force pushed to http-auth branch. Please check the changes.

• Added custom overlay for http auth
• Handled Firefox spec
• Made changes to credentialsFound as discussed previously

What we need to handle also:

• Pressing Cancel in popup should stop request
• Show error after submitting invalid credentials
• Remember new credentials after invalid ones were submitted
• Also I think it's a good idea to have a caption above the form (something like Authorizing at http://example.org), so users will understand what's going on. Maybe popup is better than overlay. At first I sticked to popup, but finished with overlay at the end.
• Handle proxy authorization
• Handle multiple http authorization at the same time

Last two issues relate to the fact that I removed a bunch of code from background/httpauth.js. For my test case everything works and sufficient. But I think I need to put some of the code back to solve last issues. Anyway I need to understand what's needed and what's not and now it's easier to add code step by step.

[limpkin]

it seems to work well... congrats! still waiting to read CI integration tests though