bit of formatting - whitespace police
- added DataView options.stableSortIdProperty option in conjunction with **stable sort** support for the DataView via mapping, which is based on https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort. Either a user provides his own stable sort mapper/unmapper helpers (see Example 4A source code) or a default stable sort mapper is applied instead.

- fixed the disappearance of the sort-asc/desc icons in the header when the header is too narrow: now the icons are `position:absolute` positioned and the header name styling is adjusted accordingly to ensure both are always shown in the header, even when it is very narrow.

- cleanup: removed unused function percentCompleteSort() from the examples
updated all examples and tests to include assert.js (required by slickgrid now)