Initial commit of sparkline for realtime graphs.

Changes:

* Add configurable time period option to module/collection.
* Convert realtime collection to matrix collection.
* Update view to render sparkline, add hover listeners.
* Remove interpolation methods from view, since these don't work well with
  sparklines (discussed this change with Phil).
* Add and update relevant styles.
* Update tests, including Cucumber tests.
Use 24 hours' worth of data.

* Refactor collection to use descending sort, since otherwise
Backdrop gives us old data. Reverse the data in parse method.
* Use 24 hours' worth of data.
* Refactor tests for new data sort order.
* Remove test for converting strings to floats - now fixed
in Backdrop.
Add final set of changes for sparklines in realtime module: add
axis labels, ensure tick formatting is appropriate.

Detailed changes:

* Add configurable 'showEndTicks' option on sparklines, allowing
  us to add axis labels, as specified by design.
* When this option is set, render an additional right-hand y-axis,
  subclassed from our existing y-axis component. Show top and bottom
  labels on these axes (agreed with design).
* Ensure realtime ticks are formatted as integers in the common
  case where the only values on the sparkline are 0 and 1.
* Remove tooltip options, since we're not using them.
* Throttle hover events to deal with dense graphs that have
  24 hours' worth of data.
* Re-render graph when collection data updates.
* Update styling for axes.
* Update tests.
Improve readability of hard-coded number
Rationalise code
Update parse method to convert strings to floats, until Backdrop
issue is fixed.

Update tests and remove float conversion elsewhere.