Add realtime axes
Refactor variable names as we're just talking about generic modules.
Axes data for realtime module.
We only need the current value and previous value.

This should drastically reduce the size of our responses.
Get the valueAttr from the yaxis key.

Assuming that the yaxis is 1dimensional for now.
Adding in formatted start_at, end_at keys for modules.
Merge pull request #22 from alphagov/feature/realtime

Realtime module