Issue #1211 Multi select when using Ctrl-Key and Shift-Key

Fix for the issue "Multiselect only if Ctrl-Key is pressed #1211" https://github.com/angular-ui/ng-grid/issues/1211

Added a variable 'modifierKeysToMultiSelect' that is set to false by default. When set to true it only allows multi select to occur when the user use the Ctrl-Key or the Shift-Key. It has no effect unless 'multiSelect' is true. 

To test this feature, set both 'multiSelect' and 'modifierKeysToMultiSelect' to true. The tutorial has been updated to allow testing this (/docs/#/tutorial/210_selection)