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)
write `TariffDiff` output to rails logger

And mute it on specs
Merge pull request #129 from bitzesty/69914024-diff-script

tariff:diff rake task for comparing data on two hosts
Add guidance for contributors
Update Rails to 3.2.18
Merge pull request #130 from alphagov/3.2.18

Update Rails to 3.2.18
Remove router rake task

This is a hangover of the old router. Trade Tariff doesn't register
against the new router (which is a good thing because it wouldn't work
as it would register in preview with a domain of
production.alphagov.co.uk because it's based on FACTER_govuk_platform)

I'm removing FACTER_govuk_platform and I stumbled across this, so I
thought I'd just delete the rake task.
Merge pull request #132 from alphagov/remove-govuk_platform

Remove router rake task
Fix for broken popup.

Missing //= require popup in application.js
Fix Popup under the page

While the popup is now visible it is under the page, add a z-index
so that it is on top of the page.