Improve diagnostics (PhantomJS logs, Readability.js messages/errors)

[nickolay]

Regarding #12, the new version of PhantomJS was no better in than the old one when it failed :)

After playing around yesterday I have a number of patches that improve diagnostics making it easier to debug problems. Not sure which of them are acceptable (e.g. is --debug output safe to include in the http://readable-proxy.herokuapp.com/ and what is the browser support of the Web UI - is the Fetch API fine to use?).

This conflicts with https://github.com/n1k0/readable-proxy/pull/20/commits/62ca359c36fd8d65e4e9d21100268ab4713da290 , but I don't understand the reason behind that change.