Tidy up tests

Adds a few additional tests to Bubble and Scatter (mainly checking for
negative radius), and factors out some test info into a beforeEach()
Merge branch 'upstream' into feature/point-series
Fix typo in init docs

Copy-paste fail
Inject PointSeries

Brings in the PointSeries class as a common base class between Scatter
and Bubbles. For now, this is just empty.
Move common functions to PointSeries

Moves rangeX, rangeY, radiusFunction and opacityFunction to the common
baseclass.

Also moves pointRadius, but deprecates it in favour of radiusFunction.
Common drawing for PointSeries

Elevates the draw method to the common PointSeries class.
This required that a few methods be renamed (bubbleData + scatterData =>
  pointData), and that a few more methods become 'public', such as the
  selector.
Use correct classValues for each series
Fix missing transitions from scatter series

Noticed this was different between the two draw methods!
Update Changelog
Fix test name typo
Organise public/internal/private variables/functions
Respond to review comments

- Marks inputs to radiusFunction and pointRadius as Function|Number
- Corrects wrong @memberof! comments
- Makes radiusFunction use d3.functor
- Updates change log to mention deprecation
Add @extends to Series subclasses.

Generates docs from the inherited superclass, marking the methods as
"inherited".
Docs tweaks
Make scatter series opaque

Defaults scatter series to complete opacity.
Fix scatter point movement

This got lost in a rebase. The parameter to selectAll needs to match
the value given to points.enter().attr('class', ...);
Merge pull request #170 from tpkelly/feature/point-series

Refactor Scatter/Bubble Series