add `tariff:diff` task

Usage:
`rake tariff:diff[host1,host2] chapter=01`
It traverses the tree beginning on the specified chapter and compares all nodes between two endpoints

If the host needs basic authentication, you can set username and password on `host1user` and `host1passwd` or `host2user` and `host2passwd`
Example:
`rake tariff:diff[https://www.gov.uk/,http://tariff.dev.gov.uk/] host2user=macool host2passwd=marioandres chapter=01`

[pivotal story](https://www.pivotaltracker.com/n/projects/488191/stories/69914024)