Add a test fo cell options.

This was missing previously.
Clarify API doc.
Make cells inherit more data from the view.

As Cells are often created in the view, it makes sense that a cell will
inherit the containing view's helpers and classname. By propagating both
helpers and view class names developers can expect to get the same
rendering engine in both their views and cells.

Refs #3926
Refs #4358
Refs cakephp/app#118
Update docs.
Make cell creation a protected method.

This will let classes using CellTrait to more easily the cell creation
process.
Correct wording.
Casing.
Adjust wording.
Merge pull request #4388 from cakephp/view-cell-helpers

3.0 Pass view class and helpers to Cells