fixes #1668 - added new options for allow column header insert / remove
TODO - tests tests tests
Merge branch 'develop' into feature/issue-1668

Conflicts:
	src/plugins/contextMenu.js
fixes #1668
merged develop + tests
fixes #1668
merged develop + tests
refs #HOT-42
refs #HOT-42
zmiany w testach
small changes
Adding tests that verify the issue #1810
Fix for issue #1810
remove console.logs
bugfix: wrong order of assignments resulted in reverse event invocation (because `bubbling` became a function, which always resolves to boolean true)
bugfix: instance.eventListeners.push must be done after `delegate`, `callback`, `bubbling` are resolved - otherwise the wrong values are saved in the array
bug fix: overwriting callback function with a function expression makes it impossible to removeEventListener to remove the original handler
bugfix: `clear` and `removeEventListener` did not work if the `delegate` argument was omitted
remove console.logs
be consistent - always use document.documentElement
bugfix: removeElementListener was used without handler/callback argument
bugfix: removeEventListener was used on an event that was never added
bugfix: eventManager.clear() was not called when Handsontable is destroyed