Show percents on 100% graph tooltip

Rather than the value as a 100percent graph the percent is the important
thing.
Allow an arbitary number of periods to be shown

Allow the setting of a duration property which is passed through to the
query model. That then dynamically works out the start date to ask
backdrop for. Can be used to show more or less than the default number
of periods on a graph.
Ensure percent is rendered while on a 100% graph
Add test to ensure duration is passed to query

Query will test to make sure that it does the right thing. We just care
that it is passed through and that it doesn't apear in the compiled url.
Standardise indenting of objects

Other objects were indented relative to the var, this wasn't so indent
it a little.
Merge pull request #162 from alphagov/tooltip-percent

Tooltip percent and arbitrary number of periods