Enhancement cellcounter/cellcounter#191

Cellcounter now stores an history of keypresses in an array, allowing ```Backspace``` key to undo the last keypress, whether it was normal or abnormal in cell type.
Update help text to include new role of ```Backspace``` key.
Remove whitespace line for easier readability
Prevent default backspace behaviour (i.e. navigate back)
Merge of master-JS changes into history feature branch
JSHint cleanup of dot notation references
Merge pull request #216 from jvc26/cellcounter/cellcounter#191

Enhancement cellcounter/cellcounter#191