Add GroupedTimeshiftCollection to timeshift data

For some graphs we want to be able to show a series timeshifted from
the other series. Add a collection which will increse the duration
required to cover all the series. Then slice the return values so the
correct parts of data are selected for each series.

Allow the grouped_timeseries module to automatically switch to this
collection if it detects configuration asking for a timeshifted series.
Add labels to timeshifted graphs

So you now the data you are looking at isn't from the current time
window.
Use dashed lines for old data so they look old
Use timeshifted graph for traffic graph
This filter isn't needed to render the graphs.
Tidy to comply with jshint
Updated a test description to make it a little clearer.
Stopped us testing config.
Updated minor typo.
Updated test to reflect what the code is doing - we're using a string in the view not a moment object.
Capitalise Directgov correctly
Merge pull request #183 from alphagov/timeshifted-graph

Timeshifted graph