Fix issue 1908 (cancel autofill by returning false from beforeAutofill).

Fix #1908. Add some tests to make sure it's fixed. Here's a jsfiddle demostrating the changes (the dialog box is not part of HOT but part of the event handler, don't worry I haven't wired something ugly into the core): http://jsfiddle.net/nL5cakvm/

Apologies if I misunderstand the the git flow workflow; I can't push directly to a feature branch in the main repo, but if this PR should be against a different branch I can do that.

Also, in all honesty, I should say I've been seeing this error at a `grunt test` off and on, both on this branch and on master, and it's not happening now, which leads me to suspect something with caching or something not my code. The tests all still pass.

```
Running "jasmine:handsontable" (jasmine) task
Testing jasmine specs via phantom
..................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................>> TypeError: 'null' is not an object (evaluating 'el.select') at
>> dist/jquery.handsontable.js:6871 
>> dist/jquery.handsontable.js:6841 
>> TypeError: 'null' is not an object (evaluating 'el.select') at
>> dist/jquery.handsontable.js:6871 
>> dist/jquery.handsontable.js:6841 
>> TypeError: 'null' is not an object (evaluating 'el.select') at
>> dist/jquery.handsontable.js:6871 
>> dist/jquery.handsontable.js:6841 
```